Systems and methods for efficiently provisioning and assuring Quality of Service (QoS) between user networks communicating over a DiffServ-enabled network, with QoS management transparency to SIP user agents. The system comprises user networks communicating via a core network, each user network having a source and destination SIP user agent respectively, a SIP proxy server between the source SIP user agent and destination SIP user agent, a Bandwidth Manager to provision a pipe between the source user network and the destination user network, wherein the pipe has a specified bandwidth, and a QoS Agent for accepting and/or rejecting a SIP session based on availability of bandwidth in the pipe; wherein the SIP proxy server is configured to forward an incoming SIP request to the QoS Agent. The method comprises provisioning a pipe between the SIP user agents or their respective user networks, and allowing/rejecting incoming SIP sessions based on the available bandwidth in the pipe.