E SIP: tel:+4687197378; SIP/2.0 Route:<sip:trf1.visitedV.net;lr;iotl=homeA-visitedA> Feature-Caps:*;+g.3gpp.loopback=<"homenetwork_A"> Content-Type: application/sdp Content-Length: (...) Roaming 239 C=IN IP4 190.1.15.1 m=audio 16789 RTP/AVP 96 97 a=visited-realm:] Va.operatorV.: . net IN IP4 192.0.2.1 49170 a=visited-realm: 2 Xal.operatorH.net IN IP4 178.15.1.1 62111 a=visited-realm:3 Ha.operatorH.net IN IP4 190.1.15.1 16789 6) The IBCF-3 instance transfers to the IBCF entity of the visited network (IBCF-4 instance) the SIP INVITE message, whose SDP message replaces the RTP stream characteristics received from the IBCF-2 instance with those reported by the TrGW-2 entity. The SIP message adds the instance 4 of the header a=visited-realm populated with the domain name of the network to which the IBCF-3 instance is connected (Xa2.operatorX.net), the IP address and the port number of the RTP, stream provided by the TrGW-2 entity for the specified domain name. INVITE SIP: tel:+4687197378; SIP/2.0 Route:<sip:trf1.visitedV.net;lr;iotl=homeA-visitedA> Feature-Caps:*;+g.3gpp.loopback=<"homenetwork-A"> Content-Type: application/sdp Content-Length: (...) C=IN IP4 179.14.1.2 m=audio 34500 RTP/AVP 96 97 a=visited-realm: 1 Va.operatorV. . net IN IP4 192.0.2.1 49170 a=visited-realm: Xal.operatorX.net IN IP4 178.15.1.1 62111 a=visited-realm: Ha.operatorH.net IN IP4 190.1.15.1 16789 a=visited-realm: Xa2.operatorX.net IN IP4 179.14.1.2 34500 240 VoLTE and ViLTE 7) Upon receipt of the SIP INVITE message, the IBCF-4 instance detects that the case of optimal routeing is possible. The IBCF-4 instance transfers to the TRF entity the SIP INVITE message, whose SDP message replaces the RTP stream characteristics of the IBCF-3 instance with those of the instance 1 of the header a=visited- realm. Instances 2 to 4 of the header a=visited-realmare deleted. INVITE SIP: tel:+4687197378; SIP/2.0 Route:<sip:trf1.visitedV.net;lr;iotl=homeA-visitedA> Feature-Caps:*;+g.3gpp.loopback=<"homenetwork-A"> Content-Type: application/sdp Content-Length: ( C