 inserts Bob's IP address instead of the phone number into the SIP URI identity of the SIP INVITE request. The S-CSCF entity adds the Route header containing the URI identity of the P-CSCF entity and iotl parameter indicating the direction of the request (homeB-visitedB). The IBCF-5 instance forwards to the IBCF (IBCF-6 instance) entity the SIP INVITE message without changing the SDP message. 234 VoLTE and ViLTE INVITE <sip:193.0.2.1@operatorY> SIP/2.0 Route: <sip:pcscf1.visitedVB.net;lr;iotl=homeB-visitedB> Content-Type: application/sdp Content-Length: (...) C=IN IP4 191.1.15.2 m=audio 12538 RTP/AVP 96 97 3) The IBCF-6 instance transfers to the P-CSCF entity the SIP INVITE message, whose SDP message replaces the RTP stream characteristics received from TrGW-3 entity with that communicated by the TrGW-4 entity. INVITE <sip:193.0.2.1@operatorY> SIP/2.0 Route: :<sip:pcscf1.visitedVB.net;lr;iotl=homeB-visitedB> Content-Type: application/sdp Content-Length: (...) C=IN IP4 193.0.2.2 m=audio 9452 RTP/AVP 96 97 4) The P-CSCF entity removes its identity from the Route header and transfers the SIP INVITE message to Bob's UA entity. INVITE <sip: :193.0.2.1@operatorY> SIP/2.0 Content-Type: application/sdp Content-Length: (...) C=IN IP4 193.0.2.2 m=audio 9452 RTP/AVP 96 97 . . . 5) Alice's UA entity generates the SIP 183 Session Progress message to the P-CSCF entity, whose associated SDP message contains the RTP stream characteristics. Roaming 235 SIP/2.0 183 Session Progress Content-Type: application/sdp Content-Length: (...) C=IN IP4 193.0.2.1 m=audio 59170 RTP/AVP 97 98 The SIP 183 Session Progress message is forwarded without changing the SDP message to the IBCF-6 instance. 6) The IBCF-6 instance forwards to the IBCF-5 instance SIP 183 Session Progress message, whose SDP message replaces the RTP stream characteristics received from the Bob's UA entity with those provided by the TrGW-4 entity. SIP/2.0 183 Session Progress Content-Type: application/sdp Content-Length: ( ) C=IN IP4 179.15.1.1 m=audio 62111 RTP/AVP 97 98 The