A system and method for allowing legacy circuit switch user equipment (CS UE) to operate via a packet switch system, such as an IP Multimedia Subsystem (IMS) system, is provided. The mobility and session control aspects of communications with the legacy CS UE is separated. A user agent is placed in the receiving node (e.g., a home node B) that acts as the SIP agent for the CS UE for session control. An interworking function is provided to allow mobility between the macro CS network and the PS (e.g., IMS) network. Hand-back and hand-in procedures with service continuity are also provided.