A design for a unified session signaling system for use in multimedia communications is disclosed. In one embodiment, a method includes interfacing, via an application interface, with an associated application and a session, tracking, via a call state/session manager, a call state and session properties across multiple calls associated with the session, managing, via a server interoperation module, registration and proxying services associated with the session, managing, via a basic SIP services module using a third party SIP stack, a basic set of SIP services associated with the application and the session, and determining and advertising, via a media negotiator module, media capabilities of devices associated with the session. The method may also include managing, via an additional SIP services module using the third party SIP stack, a set of additional services associated with the session.