User equipment calls using a predetermined call function based on disabled operated modes

Abstract

To allow user equipment (UE) to place a call on a cellular network, the UE first determines whether an operating mode associated with a fallback network is disabled. Based on the operating mode associated with the fallback network being disabled, the UE then transmits a call registration request that omits call function data to the cellular network. After receiving an acknowledgement from the cellular network, the UE removes data from the acknowledgement indicating which call functions are supported by the cellular network. Based on the modified acknowledgement, the UE then initiates a call on the cellular network using a predetermined call function.

Description

USER EQUIPMENT CALLS USING A PREDETERMINED CALL FUNCTION BASED ON DISABLED OPERATED MODESBACKGROUND

[0001] To allow user equipment (UE) to place calls to other UE, a cellular network typically includes one or more base stations each configured to connect to UEs using one or more radio access technologies such as 3rd generation (3G), 4th generation (4G), and 5th generation (5G) radio access technologies. Additionally, the cellular network includes a network switching subsystem configured to route network messages between the cells of the cellular network so as to facilitate a call between two UEs connected to the cells of the cellular network. Further, such a network switching subsystem is configured to support various call functions such as voice over New Radio (VoNR), Evolved Packet Switched Fallback (EPSFB), voice over Long Term Evolution (VoLTE), and the like based on the architecture of the network switching subsystem. To enable a UE to utilize cellular networks each having network switching subsystems with different architectures such as evolved packet core (EPC) architectures or service-based architectures (SBAs), some UEs include multiple operating modes each configured to allow the UE to communicate with a network switching subsystem having a corresponding architecture. Using a corresponding operating mode, a UE is then able to place calls over a cellular network using the call functions supported by the network switching subsystem of the cellular network.BRIEF DESCRIPTION OF THE DRAWINGS

[0002] The present disclosure may be better understood, and its numerous features and advantages are made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items.

[0003] FIG. 1 is a block diagram of a cellular networking framework including a user equipment (UE) configured to place calls using a predetermined call function based on an operating mode associated with a fallback network being disabled, in accordance with some embodiments.

[0004] FIG. 2 is a block diagram of an example LIE configured to place calls using a predetermined call function based on an operating mode associated with a fallback network being disabled, in accordance with embodiments.

[0005] FIGs. 3 and 4 together present a signal flow diagram of an example operation for placing calls using a predetermined call function based on an operating mode associated with a fallback network being disabled, in accordance with embodiments.

[0006] FIG. 5 is a flow diagram of an example method for placing calls using a predetermined call function based on an operating mode associated with a fallback network being disabled, in accordance with embodiments.

[0007] FIG. 6 is a flow diagram of an example method for adding call function capability data back to call registration requests, in accordance with embodiments.SUMMARY OF EMBODIMENTS

[0008] Techniques and systems described herein are directed to allowing UE to place calls on a cellular network using a predetermined call function when one or more operating modes associated with a fallback network are disabled. According to an example embodiment, a method includes, based on an operating mode associated with a fallback network of the UE being disabled, transmitting, to a cellular network, a call registration request omitting data indicating each call function supported by the UE. Further, the method may include initiating a call on the cellular network using a predetermined call function.

[0009] In embodiments, the method can also include, based on camping on a cell of the cellular network using 5th generation new radio protocols, transmitting, to the cellular network, a registration request indicating an S1 mode of the UE is disabled. The operating mode of the UE associated with the fallback network may include the S1 mode. Further, the method can include removing, from a session initiation protocol (SIP) register message, a multimedia telephony feature tag or audio feature tag to produce the call registration request omitting data indicating each call function supported by the UE. The method may also include, based on receiving a call registration acknowledgement from an internet protocol multimedia subsystem (IMS)of the cellular network, modifying the call registration acknowledgment such that the modified call registration acknowledgment omits data indicating each call function supported by the IMS of the cellular network. Initiating the call on the cellular network using the predetermined call function may be in response to receiving the modified call registration acknowledgment omitting data indicating each call function supported by the cellular network.

[0010] Additionally, the method can include, based on the operating mode associated with the fallback network of the UE being enabled after being disabled, transmitting, to the cellular network, a second call registration request including the data indicating each call function supported by the UE. As well, the method may include initiating a second call on the fallback network using a call function associated with the operating mode. The method can also include generating a SIP register message that includes a multimedia telephony feature tag or audio feature tag to produce the second call registration request including the data indicating each call function supported by the UE. Further, the method can include disabling the operating mode of the UE associated with the fallback network based on a network message received from a second cellular network. Additionally, the method can include, based on an operation preference indicating a second call function, initiating a second call on the cellular network using the second call function. Initiating the call on the cellular network using the predetermined call function may be in response to the second call failing.

[0011] In another example embodiment, a method includes, based on camping on a cell of a cellular network, reporting to the cellular network that an operating mode of the UE associated with a fallback network is unavailable. Additionally, the method can include removing data indicating each call function supported by an internet protocol multimedia subsystem (IMS) of the cellular network from a call registration acknowledgment received from the cellular network. Removing such data can produce a modified call registration acknowledgment that omits data indicating each call function supported by the IMS of the cellular network. Further, the method may include, based on the modified call registration acknowledgment omitting data indicating each call function supported by the IMS of the cellular network, initiating a call on the cellular network using a predetermined call function.

[0012] In embodiments, the method can include, based on the operating mode of the UE associated with the fallback network being disabled, removing a multimedia telephony feature tag or audio feature tag from a session initiation protocol (SIP) message. As well, the method can include transmitting the SIP message omitting the multimedia telephony feature tag or audio feature tag to the cellular network. Further, the method can also include, based on the operating mode of the UE associated with the fallback network being enabled after being disabled, generating a second SIP message including the multimedia telephony feature tag or audio feature tag. Additionally, the method may include transmitting the second SIP message including the multimedia telephony feature tag or audio feature tag to the cellular network. As well, the method may include, based on receiving a second call registration acknowledgment including data indicating each call function supported by the IMS of the cellular network, initiating a second call on the cellular network using a call function associated with the operating mode of the UE associated with the fallback network.

[0013] The method can additionally include, based on an operation preference indicating a second call function, initiating an initial call on the cellular network using the second call function. Initiating the call on the cellular network using the predetermined call function may be in response to the initial call failing. Further, the method can include disabling the operating mode associated with the fallback network based on one or more operation preferences of the UE. As well, the method can include enabling the operating mode associated with the fallback network after being disabled based on a predetermined amount of time elapsing.

[0014] According to another example embodiment, a method includes, based on camping on a cell of a cellular network using 5th generation new radio protocols, transmitting, to the cellular network, a registration request indicating an S1 mode of the UE is disabled. Additionally, the method can include transmitting, to the cellular network, a session initiation protocol (SIP) register message that omits a multimedia telephony feature tag or audio feature tag. Further, the method may include, based on receiving a call registration acknowledgement from an internet protocol multimedia subsystem (IMS) of the cellular network, modifying the call registration acknowledgment such that the modified call registration acknowledgment omits dataindicating each call function supported by the IMS of the cellular network. Also, the method can include initiating a call on the cellular network using a predetermined call function.

[0015] Generally, in embodiments, the cellular network can be a 5th generation standalone network. Further, the predetermined call function can be a voice over internet protocol (VoIP) call function. Additionally, the UE may not not support voice over new radio (VoNR). Also, the fallback network can be a non-standalone network or a 4G network.

[0016] In an additional example embodiments, an apparatus can include a UE. Such an UE may include one or more processors and a memory coupled to the one or more processors and storing executable instructions configured to manipulate the one or more processors to perform one or more of the methods disclosed herein.

[0017] In other example embodiments, a UE can include a modem and a connection circuitry. This connection circuitry may be configured to one or more of the methods disclosed herein.DETAILED DESCRIPTION

[0018] To allow a UE attached to a cellular network to place a call to another UE, the cellular network includes multiple base stations (e.g., cells) each communicatively coupled to one or more other UEs. Further, the cellular network includes a network switching subsystem (e.g., core) that routes data between the base stations such that voice calls between the UE and the other UE are facilitated using a corresponding call function (e.g., VoNR, voice over internet protocol (VoIP), VoLTE, EPSFB, circuit switching (CS)) associated with the network type (e.g., 3G network, 4G network, 5G network) of the core. For example, a core associated with a 3G network routes data such that voice calls use VoIP or CS call functions, a 4G network routes data such that voice calls use VoIP or VoLTE call functions, and a 5G network routes data such that voice calls use VoIP, EPSFB, or VoNR call functions.

[0019] However, these different types of cellular networks (e.g., 5G networks, 4G networks, 3G networks) each use corresponding communication interfaces to send messages between the base stations of the cellular network and the core of thecellular network, the base stations of the cellular network and a UE, the core of the cellular network and a UE, or any combination thereof. These communication interfaces, for example, each dictate the syntax of messages, how messages are sent between the base stations of the cellular network and the core of the cellular network, how messages are sent between the base stations of the cellular network and a UE, how messages are sent between the core of the cellular network and a UE, or any combination thereof. For example, some 5G networks include an N1 communication interface that indicates how messages are sent between a UE and a 5G core (e.g., Access and Mobility Management Function (AMF)) configured to offer 5G call functions such as VoIP or VoNR calls. As another example, some 5G networks include an S1 interface that indicates how messages are sent between base stations connected to a UE via 5G communication protocols and a 4G core (e.g., evolved packet core (EPC)) configured to provide 4G call functions such as VoIP calls or EPSFB calls.

[0020] To enable a UE to connect to cellular networks each using corresponding communication interfaces, the UE is configured to operate in different operating modes each corresponding to a respective communication interface used by the cellular networks. As an example, the UE is configured to operate in a standalone operating mode (e.g., N1 mode) during which the UE is connected to a base station via a first radio access technology (e.g., 5G radio access technology) and sends messages to the cellular network based on a first communication interface (e.g., N1 interface) of the cellular network. As another example, the UE is configured to operate in a non-standalone operating mode (e.g., S1 ) mode during which the UE is connected to a base station via the first radio access technology (e.g., 5G radio access technology) and sends messages to the cellular network based on a second communication interface (e.g., S1 interface). As yet another example, the UE is configured to operate in an LTE operating mode during which the UE is connected to a base station via a 4G radio access technology and sends messages to the cellular network based on a communication interface associated with a 4G network. In this way, the UE is configured to attach to different cellular networks having respective cores each offering different call functions (e.g., VoNR, EPSFB, VoIP, VoLTE).

[0021] Additionally, to enable LIE not supporting VoNR to place a call on a 5G network, some 5G networks support EPSFB call functions wherein a UE is rerouted to the core of a fallback network (e.g., 5G non-standalone network, 4G network) that supports call functions associated with 4G networks such as VoLTE. After being rerouted, the UE then places a call using the call functions supported by the core of the fallback network (e.g., using 4G call functions). As such, to be enabled to use such EPSFB call functions on the 5G network, the UE includes a non-standalone operating mode, LTE operating mode, or both that allow the UE to place a call on the fallback cellular network. However, certain conditions arise that disable one or more operating modes associated with the fallback network. For example, certain network messages received from a cellular network such as the unavailability of certain cellular functions (e.g., 4G functions), detach requests, and the like increase the likelihood that a corresponding operating mode associated with a fallback network (e.g., non-standalone operating mode, LTE operating mode) is disabled. As another example, certain operation preferences of the UE, such as new-radio-only call preferences, increase the likelihood that a corresponding operating mode (e.g., non- standalone operating mode, LTE operating mode) of the UE is disabled. Due to the operating mode associated with a fallback network being disabled, the UE reports to a 5G network supporting EPSFB call functions that such an operating mode is unavailable, preventing the UE from using the EPSFB call function of the 5G network. As an example, based on a non-standalone operating mode, LTE operating mode, or both of the UE being disabled, the UE reports to a 5G network supporting EPSFB call functions that a non-standalone operating mode or LTE operating mode, respectively, is unavailable. Because the UE has not indicated an available operating mode that supports call functions of the cellular networks to which the UE would be rerouted (e.g., fallback networks), the 5G network transmits an error message to the UE. As such, the UE is not able to use the EPSFB call function of the 5G network.

[0022] In light of this, systems and techniques disclosed herein are directed toward allowing a UE to place calls on a cellular network using a predetermined call function based on one or more operating modes associated with a fallback network being disabled. For example, the UE first determines whether a certain operating mode (e.g., non-standalone operating mode, LTE operating mode) associated with a fallback network is disabled for the UE. As an example, based on certain messages(e.g., detach messages, error messages) received from a cellular network, operation preferences of the UE, or both, the UE determines whether a certain operating mode associated with a fallback network is disabled. After determining the operating mode associated with a fallback network is disabled, the UE connects to a cell of a cellular network using a first radio access technology (e.g., 5G radio access technology). The UE then begins to register itself on the cellular network by having a modem of the UE send one or more registration messages to the cell of the cellular network indicating user information, carrier information, UE capability data, available operating modes, or any combination thereof associated with the UE. After being registered on (e.g., camped on) the cellular network, the UE then initiates a call by first generating a call registration request (e.g., session initiation protocol (SIP) register) that omits data indicating which call functions (e.g., VoNR call functions, VoLTE call functions, EPSFB call functions) are supported by the UE. That is to say, the UE removes data from the call registration request indicating which call functions are supported by the UE before the modem of the UE sends the call registration request to the cellular network. In response to the modem of the UE sending the call registration request, the modem of the UE receives a call registration acknowledgment from the cellular network indicating that the UE is enabled to place calls and indicating the call functions supported by the core of the cellular network. The UE then removes data from the call registration acknowledgment indicating which call functions are supported by the core of the cellular network and passes the modified call registration acknowledgment to an application programming interface (API) associated with voice calling. The UE then initiates a phone call over the cellular network using a predetermined call function such as VoIP. In this way, the UE is still enabled to use the predetermined call function to place a call on a 5G network supporting EPSFB call functions even when certain conditions arise that disable one or more operating modes, helping to improve user experience.

[0023] FIG. 1 presents a cellular networking framework 100 including a UE 102 configured for placing calls using a predetermined call function in response to a disabled operating mode, in accordance with embodiments. To this end, cellular networking framework 100 includes cellular network 108 associated with one or more cellular carriers (e.g., cellular providers). Cellular network 108 is configured to transmit and receive one or more network messages to and from UE 102 andincludes, for example, a 3G network 155, 4G network 145, or 5G network 135. Such a 3G network 155, for example, includes a Global System for Mobile Communication (GSM) cellular network, a Universal Mobile Telecommunications System (UMTS) cellular network, and the like. A 4G network 145, as an example, includes a Fourth Generation (4G) Long-Term Evolution (LTE) cellular network. Additionally, a 5G network 135 includes, for example, a Fifth Generation (5G) New Radio (NR) cellular network, 5G Technical Forum (5GTF), a 5G Special Interest Group cellular network (5G-SIG), or the like. According to some embodiments, cellular network 108 is a 5G network 135.

[0024] According to embodiments, cellular network 108 is configured to transmit and receive, from one or more UEs, network messages that include, for example, telephone calls, VoIP calls, VoLTE calls, EPSFB calls, VoNR calls, CS calls, Short Message Service (SMS) messages, Multimedia Messaging Service (MMS) messages, authentication messages, registrations messages, update messages, attach messages, service request messages, or any combination thereof, to name a few. To facilitate the transmission and reception of network messages between cellular network 108 and UE 102, cellular network 108 includes one or more cells 110 configured to receive and transmit network messages to and from UE 102. Each cell 110 includes, for example, mobile base station towers (e.g., “cell towers”), antennae, transmitters, receivers, digital signal processors, control electronics, Global Positioning System (GPS) receivers, base transceiver stations, or any combination thereof, configured to receive and transmit network messages to and from UE 102. Though the example embodiment illustrated in FIG. 1 presents the cellular network 108 having three cells 110-1 , 110-2, and 110-3, in other embodiments, cellular network 108 may include any number of cells 110 to facilitate the reception and transmission of network messages to and from UE 102.

[0025] To enable UE 102 to place voice calls over cellular network 108, UE 102 is configured to transmit and receive network messages from cellular network 108 and includes, for example, a compute-enabled phone (“smartphone”), a tablet computer, a personal computer, a laptop computer, a compute-enabled wearable device (e.g., smartwatch, fitness tracker, smart bracelet), an Internet of Things (loT) device, a router, a mobile hotspot device, an automotive device, a manufacturing device, orany combination thereof, to name a few. To transmit and receive network messages from cellular network 108, UE 102 includes or is otherwise connected to modem 112. Modem 112 includes circuitry configured to transmit or receive network messages using one or more radio access technologies. Such radio access technologies include a connection method or protocol used to communicatively couple UE 102 to one or more cellular networks 108, for example, 2G network connections (e.g., Global System for Mobile Communications (GSM) protocols, code division multiple access (CDMA) protocols, general packet radio service (GPRS) protocols, enhanced data rates for GSM evolution (EDGE) protocols), 3G network connections (e.g., Universal Mobile Telecommunications System (UMTS) protocols, wideband code division multiple access (W-CDMA) protocols, CDMA-200 protocols, high-speed packet access (HSPA) protocols), 4G network communications (e.g., Long Term Evolution (LTE) protocols, Worldwide Interoperability for Microwave Access (WiMAX) protocols), 5G network communications (e.g., 5G New Radio (NR) protocols, 5G Technical Forum (5GTF) protocols, 5G Special Interest Group (5G-SIG) protocols), or any combination thereof. For example, modem 112 is configured to transmit and receive network messages from cellular network 108 using 5G network communications.

[0026] To support multiple radio access technologies, modem 112 includes two or more instances of radio access technology (RAT) circuitry 103 each configured to support a corresponding radio access technology. For example, each instance of RAT circuitry 103 includes a radio configured to communicatively couple to a cell 110 of cellular network 108 using a corresponding radio access technology (e.g., 3G radio access technology, 4G radio access technology, 5G radio access technology. Referring to the embodiment presented in FIG. 1 , as an example, a first instance of RAT circuitry 0 114-1 is configured to support a first radio access technology (e.g., 4G radio access technology) and a second instance of RAT circuitry N 114-N is configured to support a second radio access technology (e.g., 5G radio access technology) that is different from the first radio access technology. Though the example embodiment presented in FIG. 1 presents modem 112 as including two instances of RAT circuitry 103, in other embodiments, modem 112 can include any number of instances of RAT circuitry 103 each supporting a corresponding radio access technology.

[0027] Additionally, modem 112 includes or is otherwise connected to connection circuitry 114 configured to generate one or more network messages, handle one or more received network messages (e.g., queue one or more received network messages, provide one or more network messages to other circuitry of UE 102), handle one or more network messages for transmission (e.g., queue one or more network messages for transmission), or any combination thereof. As an example, connection circuitry 114 includes or is otherwise connected to telephony circuitry 106 configured to generate, handle, or both one or more network messages for transmission to cellular network 108 for placing one or more voice calls to one or more other UEs. Additionally, UE 102 includes application programming interface (API) 104 configured to communicate with one or more applications (not shown for clarity) each configured to place voice calls to one or more other UEs by issuing instructions, commands, or both to connection circuitry 114, modem 112, telephony circuitry 106, or any combination thereof.

[0028] In embodiments, UE 102 is configured to place one or more voice calls to one or more other UE also camped on a cell 110 of cellular network 108. For example, UE 102 is configured to use one or more voice call protocols to place one or more calls over cellular network 108. Such voice call protocols, for example, indicate how network messages are to be generated and transmitted by UE 102 such that UE 102 is enabled to place a voice call over cellular network 108 using a corresponding call function. As an example, these voice call protocols indicate the syntax, timings, structures, and contents of network messages that are to be generated and transmitted to cellular network 108 so as to place a voice call using a corresponding call function. In embodiments, UE 102 is configured to support VoNR call protocols 105 (e.g., communication protocols associated with VoNR calls), VoIP call protocols 115 (e.g., communication protocols associated with VoIP calls), EPSFB call protocols 165 (e.g., communication protocols associated with EPSFB calls), VoLTE call protocols 175 (e.g., communication protocols associated with VoLTE calls), CS call protocols 125 (e.g., communication protocols associated with CS calls), or any combination thereof.

[0029] To place a call to another UE using one or more call protocols (e.g., VoNR call protocols 105, VoIP call protocols 115, EPSP call protocols 165, VoLTE callprotocols 175, CS call protocols 125), UE 102 is configured to camp on a cell 110 of cellular network 108 using a first radio access technology such that UE 102 is communicatively coupled to cellular network 108 (e.g., a core of cellular network 108). As an example, UE 102 is configured to camp on a cell 110 of cellular network 108 using a 5G radio access technology. To camp on the cell 110, the modem 112 of UE 102 transmits one or more network messages to the cell 110 indicating a request for registration, user identification data associated with UE 102, carrier information associated with UE 102, available operating modes of UE 102, or any combination thereof. Cellular network 108 then authenticates UE 102 based on the network messages indicating a request for registration, user identification data associated with UE 102, carrier information associated with UE 102, or any combination thereof and transmits a registration acknowledgment to UE 102 indicating that UE 102 is registered with cellular network 108. After camping on the cell 110, UE 102 then generates a call registration request message that includes location data of UE 102 (e.g., an address of UE 102), user identification data associated with UE 102, the call functions supported by UE 102, or any combination thereof. As an example, UE 102 generates a SIP register message that includes data associating user identification data of UE 102 with location data (e.g., an IP address) of UE 102 and data indicating the call functions supported by UE 102. The modem 112 UE 102 then transmits the call registration request to cellular network 108. Based on receiving the call registration request, cellular network 108 generates and sends a call registration acknowledgment to UE 102 indicating the call functions (e.g., VoNR, VoLTE, VoIP, CS) supported by cellular network 108. UE 102 then provides the call registration acknowledgment to API 104 which then uses a call function indicated in the call registration acknowledgment to place a call. For example, using telephony circuitry 106 and supported call protocols (e.g., VoNR call protocols 105, VoIP call protocols 115, EPSFP call protocols 165, VoLTE call protocols 175, CS call protocols 125), API 104 generates one or more network messages associated with placing a call with a call function indicated in the call registration acknowledgment. The modem 112 then transmits these network messages to a cell 110 of cellular network 108 so as to place a call to another UE using the indicated call function.

[0030] After receiving these network messages to place a call, the cell 110 of cellular network 108 forwards these network messages to a core (e.g., networkswitching subsystem) of the cellular network 108. Such a core (not shown for clarity), for example, is configured to route messages between the cells 110 of cellular network 108 so as to provide those network messages to one or more UEs camped on one or more cells 110 of cellular network 108. In embodiments, the core of cellular network 108 is configured to route network messages between the cells 110 of cellular network 108 so as to provide a certain call function (e.g., VoNR calling, VoIP calling, EPSFB calling, VoLTE calling, CR calling) based on the network type (e.g., 3G network 155, 4G network 145, 5G network 135) associated with the core. For example, a core of cellular network 108 associated with a first network type (e.g., 4G network 145) is configured to provide a first call function (e.g., VoIP, VoLTE), and a core of a cellular network 108 associated with a second network type (e.g., 5G network 135) is configured to provide a second call function (e.g., VoIP, VoNR) that is different from the first call function.

[0031] To help implement these call functions of the core of cellular network 108, cellular network 108 includes a communication interface (not shown for clarity) including circuitry configured to facilitate the transmission and reception of network messages between the cells 110 and core of cellular network 108 using one or more communication protocols, between UE 102 and the core of the cellular network 108, between UE 102 and the cells 110 of cellular network 108, or any combination thereof. As an example, in embodiments cellular network 108 includes one or more cells 110 each configured to communicatively couple to one or more UEs via a radio access technology (e.g., 5G radio access technology) associated with a first network type (e.g., 5G) and a core configured to provide call functions also associated with the first network type (e.g., VoNR call functions). As used herein such a network is also referred to as a “standalone network.” To facilitate communication within a 5G standalone network, for example, cellular network 108 includes a first communication interface (e.g., N1 interface) that provides for communication between UE 102 and a core of a 5G standalone cellular network. As another example, in embodiments, cellular network 108 includes one or more cells 110 each configured to communicatively couple to one or more UEs via a radio access technology (e.g., 5G radio access technology) associated with a first network type (e.g., 5G) and a core configured to provide call functions associated with the second network type (e.g., 4G), such as EPSFB call functions. As used herein such a network is also referred toas a “non-standalone network.” To facilitate communication within a 5G non- standalone network, a 4G network 145 (e.g., a 4G standalone network), or both cellular network 108 includes a second communication interface (e.g., S1 interface) that provides for communication between the cells 110 of cellular network 108 and a 4G core of the cellular network 108.

[0032] In embodiments, UE 102 is configured to connect to and use the call functions of various cellular networks 108 such as 5G standalone networks, 5G non- standalone networks, 4G networks 145, and the like. For example, according to embodiments, UE 102 is configured to operate in multiple operating modes each corresponding to a respective radio access technology and communication interface used by a 5G standalone network, 5G non-standalone network, 4G network 145, and the like. As an example, in embodiments, UE 102 is configured to operate in a first operating mode (e.g., 5G standalone mode, N1 mode) during which UE 102 is configured to camp on a cell 110 of cellular network 108 using a radio access technology associated with a first network type (e.g., 5G) and to send messages to a core of the cellular network associated with the first network type using a corresponding communication interface (e.g., N1 interface). As another example, UE 102 is configured to operate in a second operating mode (e.g., non-standalone mode, S1 mode) during which UE 102 is configured to camp on a cell 110 of cellular network 108 using a radio access technology associated with a first network type (e.g., 5G) and to send messages to a core of the cellular network associated with a second network type (e.g., 4G) using a corresponding communication interface (e.g., S1 interface). As yet another example, UE is configured to operate in a third operating mode (e.g., LTE operating mode, S1 mode) during which UE 102 is configured to camp on a cell 110 of a 4G network 145 using a radio access technology associated with a 4G network 145 to send messages to a core of the 4G network 145 using a corresponding communication interface (e.g., S1 interface).

[0033] According to some embodiments, UE 102 does not support VoNR call functions (e.g., does not include VoNR call protocols 105) but is configured to connect to a 5G network 135 using a 5G radio access technology. To this end, the 5G network 135 supports an EPSFB call function that reroutes UE 102 to the core of a second cellular network 108, also referred to herein as a “fallback network.” Forexample, the EPSFB call function of a 5G network 135 reroutes UE 102 to a fallback network (e.g., non-standalone network, 4G network 145) that allows UE 102 to place a call using a call function (e.g., VoLTE call function) supported by the core of the fallback network. As such, to place a call on a fallback network, UE 102 includes an operating mode (e.g., non-standalone operating mode, LTE operating mode) associated with the fallback network that allows UE 102 to use the call functions offered by the core of the fallback network. However, certain conditions arise that disable one or more operating modes associated with the fallback network. For example, certain network messages received from cellular network 108 increase the likelihood that an operating mode associated with the fallback network is disabled.

[0034] As an example, in embodiments, UE 102 is configured to detach from a first cellular network 108 (e.g., 4G network 145) based on receiving a network message indicating a request for a detachment with no re-attachment required. Due to this message indicating that a re-attachment is not required, the likelihood of UE 102 disabling the operating mode associated with the first cellular network (e.g., S1 mode) is increased. As another example, according to embodiments, UE 102 is configured to receive a network message (e.g., network access server (NAS) error message) from cellular network 108 that causes certain operating modes associated with network types indicated in the network message to be disabled. Further, certain operation preferences of UE 102 increase the likelihood that an operating mode associated with a fallback network is disabled. For example, in embodiments, an operation preference of UE 102 indicates that only a certain call function (e.g., VoNR) is to be used when placing calls. Due to this operation preference, the likelihood that UE 102 disables the operating mode associated with a network type (e.g., non- standalone network, 4G network 145) that provides different call functions is increased. In embodiments, based on an operating mode associated with a fallback network of UE 102 being disabled, UE 102 reports to cellular network 108 that the disabled operating mode is unavailable. For example, while camping on a cell 110, UE 102 transmits a network message to a 5G network 135 supporting an EPSFB call function. Such a network message indicates user information associated with UE 102 and the available operating modes of UE 102 (e.g., indicating that the disabled operating mode associated with the fallback network is unavailable). UE 102 then generates a call initiation request on the 5G network 135 using an EPSFB callfunction. However, because UE 102 previously indicated that the operating modes associated with fallback networks (e.g., non-standalone networks, 4G networks 145) are not available, the 5G network 135 responds to the call initiation request with an error message, preventing UE 102 from placing a call.

[0035] As such, according to embodiments, UE 102 is configured to place calls on cellular network 108 (e.g., a 5G network 135 supporting EPSFB call functions) using a predetermined call function based on an operating mode associated with a fallback network being disabled. For example, UE 102 is configured to place calls on a 5G network 135 using a predetermined call function (e.g., VoIP) even when the non- standalone operating mode, LTE operating mode, or both of UE 102 are disabled. To this end, UE 102 is configured to first determine whether an operating mode associated with a fallback network (e.g., 5G non-standalone network, 4G network 145) has been disabled. As an example, based on certain network messages (e.g., detach messages, error messages) received from a cellular network, operation preferences of UE 102, or both, UE 102 determines whether a certain operating mode associated with a fallback network is disabled. Based on the operating mode associated with the fallback network being disabled, UE 102 then camps on a cell 110 of cellular network 108 (e.g., 5G network 135) using, for example, a 5G radio access technology. Once UE 102 has camped on the cell, UE 102 then generates a modified call registration request that omits data indicating which call functions, call protocols (e.g., VoIP protocols 115, EPSFB call protocols 165, VoLTE call protocols 175, OS call protocols 125), or both are supported by UE 102. That is to say, UE 102 generates a modified call registration request that does not indicate which call functions, call protocols, or both are supported by UE 102. As an example, UE 102 generates a SIP register that omits one or more tags (e.g., multimedia telephony feature tag, audio feature tag) indicating which call functions are supported by UE 102 (e.g., which call protocols UE 102 supports). The modem 112 of UE 102 then transmits the modified call registration request to cellular network 108.

[0036] Based on receiving the modified call registration request, cellular network 108 transmits a call registration acknowledgment to UE 102 indicating the call functions (e.g., VoNR, EPSFB) that are not allowed by the core of cellular network 108. UE 102 then modifies the call registration acknowledgment by removing thedata from the call registration acknowledgment indicating the call functions (e.g., VoNR, EPSFB) that are not allowed by the core of cellular network 108. UE 102 then provides the modified call registration acknowledgment to API 104 which is configured to initiate a call using one or more predetermined call functions. For example, API 104 initiates a call using a VoIP call function. As another example, based on an operation preference indicating a priority for a first predetermined call function (e.g., CS call), API 104 first initiates a call using the first predetermined call function. In response to the call using the first predetermined call function failing, API 104 then initiates a second call using a second predetermined call function (e.g., VoIP). By initiating a call using a predetermined call function in this way, UE 102, while not supporting VoNR, is enabled to place a call on a 5G network 135 even when one or more operating modes associated with a fallback network (e.g., non- standalone operating mode, LTE operating mode) have been disabled. For example, because UE 102 removes the call functionality data from the call registration request and call registration acknowledgment, cellular network 108 (e.g., a 5G network 135) does not determine that UE 102 has operating modes disabled that support the fallback network to which UE 102 would be rerouted during an EPSFB call function. As such, cellular network 108 does not issue an error network message and allows the call to be placed. Due to this, even when certain conditions arise that disable the operating modes associated with fallback networks, UE 102 is still enabled to place a call on a 5G network 135 which improves user experience.

[0037] Referring now to FIG. 2, an example UE 200 configured for placing calls using a predetermined call function based on an operating mode associated with a fallback network being disabled is presented, in accordance with embodiments. In embodiments, example UE 200 does not support VoNR call function (e.g., does not support VoNR call protocols 105) and is implemented in cellular networking framework 100 as UE 102. Further, standalone network 224 is implemented in cellular networking framework 100 as cellular network 108. According to embodiments, standalone network 224 includes one or more cells 110 each configured to communicatively couple to one or more UEs via a first radio access technology (e.g., 5G radio access technology) associated with a first network type (e.g., 5G network 135). Additionally, standalone network 224 includes a core also associated with the first network type (e.g., 5G network 135) and configured toprovide a first call function (e.g., VoIP, VoNR, EPSFB). As an example, standalone network 224 includes a core having an internet protocol multimedia subsystem (IMS) 222 configured to provide VoIP call functions, VoNR call functions, EPSFB call functions, or any combination thereof. In some embodiments, during an EPSFB call function, example UE 200 is configured to initiate a call on standalone network 224 and then be rerouted to a fallback network (e.g., non-standalone network, 4G network 145) which then places the call for example UE 200.

[0038] To help enable UE 200 to transmit and receive network messages from standalone network 224, example UE 200 is configured to operate in a standalone operating mode (e.g., 5G standalone mode, N1 mode) 218. While in the standalone operating mode 218, connection circuitry 114 of example UE 200 is configured to first camp on a cell of standalone network 224 using a first radio access technology (e.g., 5G radio access technology). Further, while in the standalone operating mode 218, example UE 200 is configured to generate, transmit, and receive network messages to and from standalone network 224 based on the communication interface (e.g., N1 interface) implemented in standalone network 224. According to embodiments, while in the standalone operating mode 218, example UE 200 is configured to place calls using one or more call functions (e.g., VoIP, VoNR) associated with the core of standalone network 224. Additionally, UE 200 includes one or more other operating modes configured to enable UE 200 to transmit and receive network messages from one or more other types of networks, such as one or more fallback networks. To this end, in embodiments, UE 200 includes a non-standalone operating mode 220, LTE operating mode 226, or both. As used herein, a non-standalone operating mode 220, LTE operating mode 226, or both is referred to, for example, as an S1 mode. While in non-standalone operating mode 220 (e.g., 5G non-standalone mode), LTE operating mode 226, or both (e.g., while in an S1 mode) connection circuitry 114 of example UE 200 is configured to generate, transmit, and receive network messages to and from a 5G non-standalone network or a 4G network 145 based on a communication interface implemented within the 5G non-standalone network or 4G network 145, respectively. Additionally, while in the non-standalone operating mode 220, LTE operating mode 226, or both (e.g., while in an S1 mode), example UE 200 is configured to place calls using one or more call functions (e.g., VoIP, EPSFB) associated with the core of the 5G non-standalone network or 4G network 145.

[0039] In embodiments, example LIE 200 is configured to place one or more calls on standalone network 224 using a predetermined call function in response to one or more disabled operating modes associated with a fallback network. For example, example UE 200 is configured to place one or more calls on standalone network 224 using a predetermined call function (e.g., VoIP) based on non-standalone operating mode 220, LTE operating mode 226, or both being disabled. To this end, UE 200 first determines whether non-standalone operating mode 220, LTE operating mode 226, or both are disabled based on one or more network messages received from cellular network 108, one or operation preferences of UE 102, or both. Such operation preferences include data indicating a preference or priority for certain call functions such as a packet-switched (PS) operation preference 205 indicating that calls should first be made, for example, using a certain call function (VoIP), a CS operation preference 215 indicating that calls should first be made using a CS call function (e.g., CS fallback (CSFB)), or both. In embodiments, as an example, based on UE 200 receiving a detach network message from a 4G network 145 indicating that no 4G re-attach is required, UE 200 determines that non-standalone operating mode 220, LTE operating mode 226, or both are disabled as attaching to a 4G network is not required. Further, based on UE 200 receiving an error network message from a cellular network indicating that 4G service is unavailable, UE 200 determines that non-standalone operating mode 220, LTE operating mode 226, or both are disabled as there is no 4G network 145 to attach to. Additionally, based on UE 200 having an NR operation preference 205, UE 200 determines that non-standalone operating mode 220, LTE operating mode 226, or both are disabled as calls are to be made using a call function (e.g., VoIP) associated with a 5G network 135.

[0040] In embodiments, after determining that non-standalone operating mode 120, LTE operating mode 226, or both are disabled, example UE 200 then camps on a cell 110 of standalone network 224 (e.g., 5G standalone network) using a first radio access technology. As an example, example UE 200 camps on a cell 110 of a standalone network 224 using a 5G radio access technology. According to embodiments, while camping on the cell 110 of standalone network 224, example UE 200 is configured to generate and transmit one or more network messages indicating a registration request, user information associated with example UE 200, the operating modes available (e.g., which operating modes are not disabled) onexample UE 200, or any combination thereof. After UE 200 has camped on a cell of standalone network 224, UE 200 is configured to generate a call registration request 225 that omits call function capability data 235. That is to say, UE 200 generates a call registration request 225 without call function capability data 235. Such a call registration request 225, for example, includes a network message having data indicating user information and location information (e.g., an address) of example UE 200. As an example, a call registration request 225 includes an SIP register message that includes data associating user identification data of example UE 200 with location data (e.g., an IP address) of example UE 200. Additionally, such call function capability data 235 indicates the call functions (e.g., VoIP, VoLTE, EPSFB, CS) supported by example UE 200. That is to say, the call functions for which example UE 200 has a corresponding set of call protocols (e.g., VoIP call protocols 115, EPSFB call protocols 165, VoLTE call protocols 175, CS call protocols 125). To generate a call registration request 225 without call function capability data 235, in embodiments, example UE 200 is configured to omit or remove one or more feature tags from call registration request 225. For example, in embodiments, UE 200 removes one or more feature tags (e.g., multimedia telephony feature tag, audio feature tag) from one or more headers of a SIP register message. After generating the call registration request 225 omitting call function capability data 235, example user equipment 200 transmits the call registration request 225 omitting call function capability data 235 to a cell 110 of standalone network 224.

[0041] Based on receiving the call registration request 225 omitting call function capability data 235, the cell 110 forwards the call registration request 225 omitting call function capability data 235 to IMS 222. IMS 222 then generates a call registration acknowledgment indicating that example UE 200 is not enabled to place calls on standalone network 224 and the call functions disallowed by IMS 222 (e.g., VoNR, EPSFB). Standalone network 224 then transmits the call registration acknowledgment to example UE 200. In response to receiving the call registration acknowledgment, example UE 200 modifies the call registration acknowledgment such that the modified call registration acknowledgment omits data indicating the call functions supported by IMS 222. That is to say, UE 200 removes data indicating the call functions supported by IMS 222 from the call registration acknowledgment. UE 200 then provides the modified call registration acknowledgment omitting dataindicating the call functions supported by IMS 222 to an API 104. The API 104 then initiates a call using IMS 222 and a predetermined call function. For example, the API 104 initiates a call using a VoIP call function. Due to the call function capability data 235 being removed from call registration request 225 and the data indicating the call functions supported by IMS 222 being removed from the call initiation acknowledgment, standalone network 224 does not detect a discrepancy between the operating modes available on example UE 200 and the call functions supported by example UE 200. For example, standalone network 224 does not determine that example UE 200 has operating modes associated with a fallback network disabled, preventing example UE 200 from placing a call using an EPSFB call function. Because standalone network 224 does not detect such a discrepancy, standalone network 224 instead allows the call to be placed with the predetermined call function. In this way, example UE 200 is enabled to place a call on standalone network 224 even when operating modes associated with a fallback network (e.g., non-standalone operating mode 220, LTE operating mode 226) are disabled.

[0042] According to some embodiments, based on receiving the modified call registration acknowledgment omitting data indicating the call functions supported by IMS 222, the API 104 is configured to first attempt to place a call over the standalone network 224 using a call function indicated in one or more operation preferences. For example, based on example UE 200 having a CS operation preference 215 indicating that calls are to be first placed using a CS call function (e.g., CSFP call function), the API 104 first attempts to place the call over the standalone network 224 using the CS call function. In response to the call using the call function indicated in one or more operation preferences failing (e.g., in response to receiving an error message from the standalone network 224), the API 104 then places a call using a predetermined call function. For example, based on the call using the call function indicated in one or more operation preferences failing, the API 104 initiates a call using a predetermined call function (e.g., VoIP).

[0043] In embodiments, certain event triggers cause an operating mode associated with a fallback network to be re-enabled (e.g., enabled after being disabled). Such event triggers include, for example, a predetermined amount of time elapsing, a functionality mode (e.g., airplane mode) switch, a reboot of example UE 200, achange in operation preferences, example UE 200 connecting to a new network (e.g., Wi-Fi network, cellular network), or any combination thereof. As an example, based on a timer representing an amount of time expiring, an operating mode associated with a fallback network (e.g., non-standalone operating mode 220, LTE operating mode 226) is re-enabled. As another example, based on an airplane mode being enabled and subsequently disabled, an operating mode associated with a fallback network is re-enabled. As yet another example, based on an operation preference changing to indicate one or more other network types, an operating mode associated with a fallback network is re-enabled. According to embodiments, example UE 200 is configured to add call function capability data 235 to subsequent call registration requests 225 based on one or more operating modes associated with a fallback network being re-enabled. For example, UE 200 is configured to add call function capability data 235 to subsequent call registration requests 225 after non-standalone operating mode 220, LTE operating mode 226, or both are re-enabled. To this end, after an event trigger has occurred and an operating mode associated with a fallback network is re-enabled, example user equipment 200 generates a call registration request 225 that includes call function capability data 235 and transmits the call registration request including call function capability data 235 to standalone network 224. Based on standalone network 224 receiving the call registration request 225 that includes call function capability data 235, IMS 222 generates a call registration acknowledgment indicating the call function (e.g., VoIP, EPSFB) supported by IMS 222. Standalone network 224 then transmits the call registration acknowledgement to example UE 200. In response to receiving the call registration acknowledgment, example UE 200 provides the call registration acknowledgment to an API 104 which then initiates a call using a call function (e.g., EPSFB) indicated in the call registration acknowledgment.

[0044] FIGs. 3 and 4 together present an example operation 300 for placing calls using a predetermined call function associated with a disabled operating mode, in accordance with embodiments. According to embodiments, example operation 300 is implemented, in part, by UE 102 that does not support VoNR (e.g., UE 102 not including VoNR call protocols 105). Referring now to FIG. 3, according to embodiments, example operation 300 first includes the modem 112 of UE 102 receiving a detach request 305 from a first cell 324, similar to or the same as a cell110, of a first cellular network similar to or the same as cellular network 108. In embodiments, as an example, the first cellular network has a first type such as 4G network 145. The detach request 305, for example, includes data indicating that UE 102 is to detach from the first cell 324 and that a re-attachment to another cell of the first cellular network is not required. Based on receiving the detach request 305, the modem 112 of UE 102 then transmits a detach acceptance 315 to the first cell 324. Such a detach acceptance 315, for example, includes data acknowledging the detach request 305. According to embodiments, because detach request 305 indicates that a re-attachment to another cell of the first cellular network (e.g., a 4G network 145) is not required, UE 102 is configured to disable an operating mode associated with the network type of the first cellular network (e.g., a fallback network). As an example, UE 102 disables a non-standalone operating mode 220, LTE operating mode 226, or both associated with a 4G network 145.

[0045] After disabling the first operating mode, UE 102 is configured to attach to a second cell 326, similar to or the same as a cell 110, of a second cellular network, similar to or the same as cellular network 108. According to embodiments, for example, the second cellular network has a first type such as a 5G standalone network (e.g., a cellular network having cells configured to connect via a 5G radio access technology and having a core associated with 5G communication protocols). To attach to the second cell 326, UE 102 first generates a registration request 325 that includes user information associated with UE 102, carrier information associated with UE 102, available operating modes (e.g., enabled operating modes) of UE 102, or any combination thereof. After generating the registration request 325, the modem 112 of UE 102 then transmits the registration request 325 to the second cell 326. Based on receiving the registration request 325, the second cellular network registers UE 102 on the second cellular network and generates a registration acceptance 335 indicating that UE 102 has been registered, access information (e.g., ports, addresses) for the second cellular network, or both. The second cellular network, via the second cell 326, then transmits the registration acceptance 335 to the modem 112 of UE 102. Further, after transmitting the registration acceptance 335 to the modem 112 of UE 102, the second cellular network transmits, via the second cell 326, a capability request 345 to the modem 112 of UE 102. Such a capability request 345, for example, includes data requesting certain capabilities of UE 102 such assupported communication protocols, supported radio access technologies, supported radio parameters (e.g., frequencies, bands), and the like.

[0046] Referring now to FIG. 4, based on receiving the capability request 345, UE 102 generates a message indicating the capability data 355 of UE 102. Such capability data 355, for example, includes the supported communication protocols, supported radio access technologies, supported radio parameters (e.g., frequencies, bands), or any combination thereof of UE 102. After UE 102 generates the message including capability data 355, the modem 112 of UE 102 transmits the message including capability data 355 to the second cell 326. According to embodiments, after the modem 112 of UE 102 has transmitted a message indicating capability data 355, example operation 300 includes UE 102 placing a call over the second cellular network. To this end, UE 102 first generates a SIP register message 365 that omits the call function capability data 235 of UE 102. For example, UE 102 removes one or more feature tags (e.g., multimedia telephony feature tags, audio feature tags) from one or more headers of SIP register message 365 so as to remove the call function capability data 235 of UE 102 from SIP register message 365. The modem 112 then transmits the SIP register message 365 omitting call function capability data 235 to the second cell 326.

[0047] Based on receiving the SIP register message 365 omitting call function capability data 235, the second cellular network generates an SIP registration acceptance 375 which includes data indicating that the UE 102 has been registered, access information (e.g., ports, addresses) for a core (e.g., IMS 222) of the second cellular network, the functions (e.g., VoIP) supported by the core of the second cellular network, or any combination thereof. The second cell 326 then transmits the SIP registration acceptance 375 to the modem 112 of UE 102. In response to receiving the SIP registration acceptance 375, UE 102 modifies the SIP registration acceptance 375 so as to remove data indicating the call functions supported by the core of the second cellular network. That is to say, UE 102 modifies the SIP registration acceptance 375 to produce a modified SIP registration acceptance 385 that no longer indicates the call functions supported by the core of the second cellular network. UE 102 then provides the modified SIP registration acceptance 385 to an API 104 of UE 102 configured to place calls. The API 104 then initiates a call over thesecond cellular network using a predetermined call function. For example, based on receiving the modified SIP registration acceptance 385, the API 104 performance a VoIP call initiation 395 which includes placing a call on the second cellular network using a VoIP call function.

[0048] Referring now to FIG. 5, an example method 500 for placing calls using a predetermined call function based on an operating mode associated with a fallback network being disabled is presented, in accordance with embodiments. In embodiments, example method 500 is implemented at least in part by UE 102, connection circuitry 114, or both. UE 102 implementing example method 500, for example, does not support VoNR call functions. At block 505 of example method 500, UE 102 is configured to determine whether a non-standalone operating mode 220, LTE operating mode 226, or both of the UE 102 is enabled. Based on the non- standalone operating mode 220, LTE operating mode 226, or both being enabled, UE 102 repeats block 505 until the non-standalone operating mode 220, LTE operating mode 226, or both are disabled. Further, based on the non-standalone operating mode 220, LTE operating mode 226, or both being disabled, UE 102 moves to block 510. As an example, based on one or more network messages received from a cellular network 108, one or operation preferences (e.g., NR operation preference 205, CS operation preference 215) of UE 102, or both, UE 102 determines that the non-standalone operating mode 220, LTE operating mode 226, or both are to be disabled. As such, at block 505, UE 102 determines that non-standalone operating mode 220, LTE operating mode 226, or both are not enabled and moves to block 510. At block 510, UE 102 camps on a cell 110 of a standalone network 224 (e.g., 5G standalone network) with a first radio access technology (e.g., 5G radio access technology). To camp on the cell 110 of the standalone network 224, for example, UE 102 generates a network message indicating user information associated with UE 102, carrier information associated with UE 102, the available operating modes of UE 102, or any combination thereof, and transmits the network message to the cell 110 of the standalone network 224. Based on the network message transmitted by UE 102, the standalone network 224 then registers UE 102 and sends a registration acknowledgment message to UE 102.

[0049] After camping on the cell 110 of the standalone network 224, at block 515, LIE 102 generates a call registration request that omits the call function capability data 235 of LIE 102. That is to say, UE 102 generates a call registration request that does not indicate the operating modes (e.g., non-standalone operating mode 220, LTE operating mode 226) available to UE 102. After generating the call registration request that omits the call function capability data 235 of UE 102, the UE transmits the call registration request that omits the call function capability data 235 of UE 102 to the cell 110 of the standalone network 224. Based on receiving the call registration request that omits the call function capability data 235 of UE 102, the standalone network 224 then generates and transmits to UE 102 a call registration acknowledgment. The call registration acknowledgment, for example, includes data indicating that UE 102 is registered to make calls on standalone network 224, access information (e.g., ports, addresses) of the core (e.g., IMS 222) of the standalone network 224, call functions (e.g., VoIP, EPSFB, VoNR) supported by the standalone network 224, or any combination thereof.

[0050] In response to receiving the call registration acknowledgment, at block 520, UE 102 is configured to modify the call registration acknowledgment such that the call registration acknowledgment omits data indicating the call functions supported by the standalone network 224. That is to say, UE 102 modifies the call registration acknowledgment so as to remove data indicating the call functions supported by the standalone network 224. At block 525, UE 102 then provides the modified call registration acknowledgment omitting data indicating the call functions supported by standalone network 224 to an API 104 configured to place a call over standalone network 224. To this end, at block 530, UE 102 first determines whether the UE 102 has a CS operation preference 215. For example, UE 102 determines whether UE 102 has an operation preference indicating that calls are first to be made using a CS call function. Based on UE 102 having a CS operation preference 215, at block 540, the API 104 of UE 102 initiates a call over the standalone network 224 using a CS call function. In response to receiving an error message from the network (e.g., standalone network 224), the CS call failing, or both, UE moves to block 535. Further, referring again to block 530, based on UE 102 not having a CS operation preference 215, UE 102 moves to block 535. At block 535, the API 104 of UE 102 initiates a callover standalone network 224 using a predetermined call function. For example, the API 104 of UE 102 initiates a call over standalone network 224 using VoIP.

[0051] Referring now to FIG. 6, an example method 600 for adding call function capability data back to call registration requests, in accordance with embodiments. According to some embodiments, example method 600 is implemented at least in part by UE 102. In embodiments, example method 600 includes, at block 605, UE 102 determining whether a non-standalone operating mode 220, LTE operating mode 226, or both been re-enabled. That is to say, UE 102 determines whether a non- standalone operating mode 220, LTE operating mode 226, or both have been enabled after previously being disabled. As an example, certain event triggers cause non-standalone operating mode 220, LTE operating mode 226, or both to be reenabled. Such event triggers include, for example, a predetermined amount of time elapsing, a functionality mode (e.g., airplane mode) switch, a reboot of UE 102, a change in operation preferences, UE 102 connecting to a new network (e.g., Wi-Fi network, cellular network), or any combination thereof. Based on an event trigger occurring, the non-standalone operating mode 220, LTE operating mode 226, or both are again enabled and UE 102 determines that the non-standalone operating mode 220, LTE operating mode 226, or both have been re-enabled. Still referring to block 605, based on UE 102 not determining that the non-standalone operating mode 220, LTE operating mode 226, or both have been re-enabled, UE 102 repeats block 605 until the non-standalone operating mode 220, LTE operating mode 226, or both have been re-enabled. Further, based on UE 102 determining that the non-standalone operating mode 220, LTE operating mode 226, or both have been re-enabled, UE 102 moves to block 610. At block 610, UE 102 camps on a cell 110 of a standalone network 224 with a first radio access technology (e.g., 5G radio access technology). To camp on the cell HO of the standalone network 224, for example, UE 102 generates a network message indicating user information associated with UE 102, carrier information associated with UE 102, the available operating modes of UE 102, or any combination thereof, and transmits the network message to the cell 110 of the standalone network 224. Based on the network message transmitted by UE 102, the standalone network 224 then registers UE 102 and sends a registration acknowledgment message to UE 102.

[0052] After camping on the cell 110 of the standalone network 224, at block 615, LIE 102 generates a call registration request that includes the call function capability data 235 of UE 102. After generating the call registration request including the call function capability data 235 of UE 102, the UE transmits the call registration request to the cell 110 of the standalone network 224. Based on receiving the call registration request including the call function capability data 235 of UE 102, the standalone network 224 generates and transmits to UE 102 a call registration acknowledgment. The call registration acknowledgment, for example, includes data indicating that UE 102 is registered to make calls on the standalone network 224, access information (e.g., ports, addresses) of the core (e.g., IMS 222) of the standalone network 224, call functions (e.g., VoIP, EPSFB, VoLTE) supported by the standalone network 224, or any combination thereof. In response to receiving the call registration acknowledgment, at block 620, UE 102 provides the call registration acknowledgment to an API 104 configured to place a call over the standalone network 224. That is to say, UE 102 provides the call registration acknowledgment indicating call functions supported by the standalone network 224 to an API 104. At block 625, the API 104 of UE 102 then places a call over the standalone network 224 using a call function indicated in the call registration acknowledgment. For example, the API 104 of UE 102 places a call over the standalone network 224 using an EPSFB function indicated in the call registration acknowledgment.

[0053] In some embodiments, certain aspects of the techniques described above may be implemented by one or more processors of a processing system executing software. The software comprises one or more sets of executable instructions stored or otherwise tangibly embodied on a non-transitory computer-readable storage medium. The software can include the instructions and certain data that, when executed by the one or more processors, manipulate the one or more processors to perform one or more aspects of the techniques described above. The non-transitory computer-readable storage medium can include, for example, a magnetic or optical disk storage device, solid-state storage devices such as Flash memory, a cache, random access memory (RAM) or other non-volatile memory device or devices, and the like. The executable instructions stored on the non-transitory computer-readable storage medium may be in source code, assembly language code, object code, orother instruction format that is interpreted or otherwise executable by one or more processors.

[0054] A computer-readable storage medium may include any storage medium, or combination of storage media, accessible by a computer system during use to provide instructions and / or data to the computer system. Such storage media can include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu-Ray disc), magnetic media (e.g., floppy disc, magnetic tape, or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. The computer- readable storage medium may be embedded in the computing system (e.g., system RAM or ROM), fixedly attached to the computing system (e.g., a magnetic hard drive), removably attached to the computing system (e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory) or coupled to the computer system via a wired or wireless network (e.g., network accessible storage (NAS)).

[0055] Note that not all of the activities or elements described above in the general description are required, that a portion of a specific activity or device may not be required, and that one or more further activities may be performed, or elements included, in addition to those described. Still further, the order in which activities are listed is not necessarily the order in which they are performed. Also, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure.

[0056] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. Moreover, the particular embodiments disclosed above are illustrative only, as the disclosed subject mattermay be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. No limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.

Claims

WHAT IS CLAIMED IS:1 . A method for a user equipment (UE), comprising : based on an operating mode of the UE associated with a fallback network being disabled, transmitting, to a cellular network, a call registration request omitting data indicating each call function supported by the UE; and initiating a call on the cellular network using a predetermined call function.

2. The method of claim 1 , further comprising: based on camping on a cell of the cellular network using 5th generation new radio protocols, transmitting, to the cellular network, a registration request indicating an S1 mode of the UE is disabled, wherein the operating mode of the UE associated with the fallback network comprises the S1 mode; and removing, from a session initiation protocol (SIP) register message, a multimedia telephony feature tag or audio feature tag to produce the call registration request omitting data indicating each call function supported by the UE.

3. The method of claim 2, further comprising: based on receiving a call registration acknowledgement from an internet protocol multimedia subsystem (IMS) of the cellular network, modifying the call registration acknowledgment such that the modified call registration acknowledgment omits data indicating each call function supported by the IMS of the cellular network.

4. The method of claim 3, wherein initiating the call on the cellular network using the predetermined call function is in response to receiving the modified call registration acknowledgment omitting data indicating each call function supported by the cellular network.

5. The method of any of claims 1 to 4, further comprising:based on the operating mode associated with the fallback network of the UE being enabled after being disabled, transmitting, to the cellular network, a second call registration request including the data indicating each call function supported by the UE; and initiating a second call on the fallback network using a call function associated with the operating mode.

6. The method of claim 5, further comprising: generating a SIP register message that includes a multimedia telephony feature tag or audio feature tag to produce the second call registration request including the data indicating each call function supported by the UE.

7. The method of any of claims 1 to 6, further comprising: disabling the operating mode of the UE associated with the fallback network based on a network message received from a second cellular network.

8. The method of any of claims 1 to 7, further comprising: based on an operation preference indicating a second call function, initiating a second call on the cellular network using the second call function.

9. The method of claim 8, wherein initiating the call on the cellular network using the predetermined call function is in response to the second call failing.

10. A method for a user equipment (UE), comprising : based on camping on a cell of a cellular network, reporting to the cellular network that an operating mode of the UE associated with a fallback network is unavailable; removing data indicating each call function supported by an internet protocol multimedia subsystem (IMS) of the cellular network from a call registration acknowledgment received from the cellular network to produce a modified call registration acknowledgment that omits data indicating each call function supported by the IMS of the cellular network; andbased on the modified call registration acknowledgment omitting data indicating each call function supported by the IMS of the cellular network, initiating a call on the cellular network using a predetermined call function.11 . The method of claim 10, further comprising: based on the operating mode of the UE associated with the fallback network being disabled, removing a multimedia telephony feature tag or audio feature tag from a session initiation protocol (SIP) message; and transmitting the SIP message omitting the multimedia telephony feature tag or audio feature tag to the cellular network.

12. The method of claim 11 , further comprising: based on the operating mode of the UE associated with the fallback network being enabled after being disabled, generating a second SIP message including the multimedia telephony feature tag or audio feature tag; and transmitting the second SIP message including the multimedia telephony feature tag or audio feature tag to the cellular network.

13. The method of claim 12, further comprising: based on receiving a second call registration acknowledgment including data indicating each call function supported by the IMS of the cellular network, initiating a second call on the cellular network using a call function associated with the operating mode of the UE associated with the fallback network.

14. The method of any of claims 10 to 13, further comprising: based on an operation preference indicating a second call function, initiating an initial call on the cellular network using the second call function.

15. The method of claim 14, wherein initiating the call on the cellular network using the predetermined call function is in response to the initial call failing.

16. The method of any of claims 10 to 15, further comprising:disabling the operating mode associated with the fallback network based on one or more operation preferences of the UE.

17. The method of any of claims 9 to 16, further comprising: enabling the operating mode associated with the fallback network after being disabled based on a predetermined amount of time elapsing.

18. A method, comprising: based on camping on a cell of a cellular network using 5th generation new radio protocols, transmitting, to the cellular network, a registration request indicating an S1 mode of the UE is disabled; transmitting, to the cellular network, a session initiation protocol (SIP) register message that omits a multimedia telephony feature tag or audio feature tag; based on receiving a call registration acknowledgement from an internet protocol multimedia subsystem (IMS) of the cellular network, modifying the call registration acknowledgment such that the modified call registration acknowledgment omits data indicating each call function supported by the IMS of the cellular network; and initiating a call on the cellular network using a predetermined call function.

19. The method of any of claims 1 to 18, wherein the cellular network is a 5th generation standalone network.

20. The method of any of claims 1 to 19, wherein the predetermined call function is a voice over internet protocol (VoIP) call function.21 . The method of any of claim 1 to 20, wherein the UE does not support voice over new radio (VoNR).

22. The method of any of claims 1 to 21 , wherein the fallback network is a non- standalone network or a 4G network.

23. An apparatus comprising: a user equipment (UE) including: one or more processors; and a memory coupled to the one or more processors and storing executable instructions configured to manipulate the one or more processors to perform the method of any of claims 1 to 22.

24. A user equipment (UE) comprising: a modem; and a connection circuitry configured to perform the method of any of claims 1 to 22.

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