SIP 100 Trying SIP 183 Session Progress SIP 183 Session Progress SETUP CALL PROCEEDING SIP INVITE SIP 100 Trying SIP INVITE SIP 100 Trying SIP 200 OK SIP 200 OK CONNECT CONNECT ACK SIP ACK SIP ACK SIP 180 Ringing SIP 180 Ringing SIP 180 Ringing SIP 180 Ringing SIP 200 OK SIP 200 OK SIP 200 OK SIP 200 OK SIP ACK SIP ACK SIP ACK SIP ACK Figure 9.6. Session establishment at terminating side MSC server and UE implementing ICS function 7) The S-CSCF entity replaces the SIP URI identity by Bob's telephone number associated with the identity of the MSC Server which recorded Bob. 254 VoLTE and ViLTE INVITE sip: 12125552222@msc.HomeB.net SIP/2.0 6), 8) Each entity responds with SIP 100 Trying response that allows for blocking the retransmission timer of the SIP INVITE request. 9), 10) Bob's UA entity responds with SIP 183 Session Progress message containing the SDP message indicating that the CS bearer is used. Bob's mobile proceeds with the establishment of the bearer in the CS network (SETUP), indicating the PSI identity as destination. 11), 13) The MSC Server entity generates a SIP INVITE request to the PSI identity. The SIP INVITE request contains an SDP offer that contains the characteristics of the audio stream provided by the MSC GW entity. 12), 14) Each entity responds with SIP 100 Trying response that allows for blocking the retransmission timer of the SIP INVITE request. The MSC server entity indicates to Bob's mobile that the media in the CS network has been established (CONNECT) and forwards the SDP message to the MSC GW entity. 15), 16) The SCC AS entity responds to the MSC server entity with SIP 200 OK message containing the SDP message received from the domain HomeA.net. 17), 18) The MSC server entity acknowledges the SIP 200 OK response with the subsequent SIP ACK request. 19), 20) Bob's UA entity responds to the SIP INVITE request received from the SCC AS entity with the SIP 180 Ringing message to indicate that Bob's phone is ringing. 23), 24) Bob's UA entity responds to the SIP INVITE request received fr