Method and apparatus for supporting circuit switched interworking

1. A method for use in a network node, the method comprising:

the network node receiving, from a first wireless transmit/receive unit (WTRU), a Session Initiation Protocol (SIP) Invite message that indicates a request for a second WTRU to participate in a combined circuit-switched (CS) and IP Multimedia Subsystem (IMS) session with the first WTRU, wherein the combined CS and IMS session between the first WTRU and the second WTRU includes a CS call at the second WTRU and a PS session at the second WTRU, and a voice over IP (VoIP) call at the first WTRU;

the network node communicating with the second WTRU to establish the combined CS and IMS session; and

the network node sending a SIP OK message to the first WTRU, wherein the SIP OK message indicates that the second WTRU has accepted the combined CS and IMS session.

2. The method of claim 1 , further comprising:

receiving data from the second WTRU associated with the IMS session of the combined session; and

communicating the data to the first WTRU.

3. The method of claim 1 , further comprising:

sending a message to initiate establishment of a packet-switched (PS) bearer for the IMS session of the combined session.

4. The method of claim 1 , further comprising:

sending a message to initiate establishment of a CS bearer for the CS session of the combined session.

5. The method of claim 1 , further comprising:

sending a SIP BYE message to the second WTRU to initiate a release of the combined session at the second WTRU.

6. The method of claim 1 , wherein the first WTRU is IMS-only capable and wherein the second WTRU is IMS-capable and CS-capable.

7. The method of claim 1 , wherein the network node is an IMS Circuit Switched Interworking Function (CSIWF).