Method for providing a subscription to an IP-based Multimedia Subsystem (IMS) for a first client of a packet-switched network is provided. The first client is identified by the same MSISDN number as a second, mobile client of a circuit-switched network. The method comprises the following steps: a) sending, by the first client, a register request to an IMS Server via the packet-switched network, b) sending, by the IMS server, a location query request to a register for the IMS user identities (HSS), c) requesting, by the register for the IMS user identities (HSS), the user subscription information of the circuit-switched network from the register for the circuit-switched network (HLR), d) sending, by the register for the circuit-switched network (HLR), to the register for the IMS user identities, the user subscription information of the second client, e) creating, by the register for the IMS user identities (HSS), a user IMS subscription information based on user subscription information of the second client.