Patent ID: 6996087

Claim:
A method for terminating a call request to a mobile unit being served by an interworking Mobile Switching Center (iMSC) in a communication system comprising a circuit-switched domain, and an Internet Protocol (IP) Multimedia Subsystem (IMS), the method comprising: sending from the mobile unit to the iMSC a circuit-switched domain registration message; sending from the iMSC to the IMS a registration indication on behalf of the mobile unit via SIP that identifies the iMSC as the current location at which the mobile unit can be reached; receiving a call intended for the mobile unit at the IMS; forwarding by the IMS the call intended for the mobile unit to the iMSC via Session Initiation Protocol (SIP) on an Mx interface, the Mx interface connecting the iMSC to a Call State Control Function (CSCF) in the IMS; sending by a Serving CSCF (S-CSCF) located in the IMS SIP messages; and perfomring by the iMSC interworking between SIP call control procedures on the Mx interface and circuit-switched call control procedures towards the mobile unit.