o 49170 RTP/AVP 96 97 230 VoLTE and ViLTE 2) The P-CSCF entity selects the IBCF entity of the visited network (IBCF-1 instance) and transfers the SIP INVITE request. The P-CSCF entity removes its uniform resource identifier (URI) in the Route header and adds that of the S-CSCF entity of the home network, indicating in the iotl parameter the direction of request (visitedA- homeA). INVITE SIP: tel:+4687197378; SIP/2.0 Route: ksip:scscf1.operatorHA.net;lr;iotl=visitedA- homeA> Content-Type: application/sdp Content-Length: (...) C=IN IP4 192.0.2.1 m=audio 49170 RTP/AVP 96 97 . . . 3) The IBCF-1 instance selects the IBCF entity of the home network (IBCF-2 instance) and transfers the SIP INVITE message, whose SDP message replaces the characteristics of RTP streams received from the Alice's UA entity by those provided by the TrGW-1 entity. INVITE SIP: tel:+4687197378; SIP/2.0 Route: k<sip:scscf1.operatorHA.net;lr;iotl=visitedA- homeA> Content-Type: application/sdp Content-Length: (...) C=IN IP4 178.15.1.1 m=audio 62111 RTP/AVP 96 97 The IBCF-2 instance forwards to the S-CSCF entity the SIP INVITE message without changing the SDP message. 4) The S-CSCF entity transfers to the IBCF-3 instance the SIP INVITE message having the following transactions: - it withdraws its URI identity in the route header; Roaming 231 - it performs the ENUM resolution on the URI identity of the destination to which it adds the iotl parameter indicating the direction of the request (homeA-homeB). INVITE <sip:+46107197378@operatorY;user=phone;iotl=homeA-homeB> SIP/2.0 Content-Type: application/sdp Content-Length: (...) C=IN IP4 178.15.1.1 m=audio 62111 RTP/AVP 96 97 When the IBCF-3 instance has received the SIP INVITE request, it generates a new SIP INVITE request to Bob's home network. 5) On receipt of the SIP 183 Session Progress message from Bob's home network, the IBCF-3 instance generates the SIP 183 Session Progress message to the S-CSCF entity whose associated SDP message contains the characteristics of the flow RTP communicated by the Tr