n. The different exchanges relating to CFU are shown in Figure 5.3. S-CSCF P-CSCF Carol SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP 181 Call Is Being Forwarded SIP 181 Call Is Being Forwarded SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP 200 OK (SDP Carol) SIP 200 OK (SDP Carol) SIP 200 OK (SDP Carol) SIP 200 OK (SDP Carol) SIP 200 OK (SDP Carol) SIP ACK SIP ACK SIP ACK SIP ACK SIP ACK Figure 5.3. CFU Service Profiles 1) The INVITE request transmitted by Alice's UA entity is received by Bob's S-CSCF entity. The INVITE message contains the SDP message of Alice's UA entity. 2) As the result of the iFC data handling is positive, the request is transmitted to the telephony application server. 3), 4) The telephony application server responds to Alice's UA entity with the 181 Call Is Being Forwarded. This message alerts Alice's UA entity that the call has been transferred. 5), 6), 7) The telephony application server generates an INVITE request, sent to Carol's UA entity. This request includes the SDP message from the INVITE request received from Alice's UA entity. 8), 9), 10) Carol's UA entity responds to the telephony application server with the message 200 OK. The response contains the SDP message of Carol's UA entity. 11), 12) The telephony application server transfers the SDP message from Carol's UA entity in the message 200 OK transmitted to Alice's UA entity. 13), 14) The ACK request from Alice's UA entity acknowledges receipt of the message 200 OK from the telephony application server. 15), 16), 17) The ACK request from the telephony application server acknowledges receipt of the message 200 OK from Carol's UA entity. The media flow is then established between Alice's and Carol's UA entities. 5.2.1.1.2. CFB Communication forwarding on busy user (CFB) consists of forwarding an incoming call when the callee (Bob) is busy. The different exchanges relating to CFB are shown in Figure 5.4. 154 VoLTE and ViLTE 1) to 5) The INVITE request transmitted by Alice's UA entity to Bob's UA enti