ty passes through the telephony application server associated with Bob's S-CSCF entity. 6), 7), 8) Bob's UA entity responds with the message 486 Busy Here. This message is intercepted by the telephony application server, which implements communication forwarding to Carol's UA entity. The following exchanges are identical to those shown for CFU (3 to 17). S-CSCF P-CSCF Carol SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP 486 Busy Here SIP 486 Busy Here SIP 486 Busy Here Figure 5.4. CFB 5.2.1.1.3. CFNR Communication forwarding on no reply (CFNR) involves forwarding an incoming call in the case of the lack of a response from the callee (Bob). The different exchanges relating to CFNR are shown in Figure 5.5. 1) to 5) The INVITE request submitted by Alice's UA entity is received by Bob's UA entity. 6) to 10) On receiving the INVITE request, Bob's UA entity responds with the message 180 Ringing. This message passes through the telephony application server which starts a timer. Service Profiles 11), 12) At the expiration of the timer, the TAS entity responds to Alice's UA entity with the message 181 Call Is Being Forwarded. This message can alert Alice's UA entity that her call was being transferred. 13), 14), 15) The TAS entity stops the ringing by sending the CANCEL request to Bob's UA entity. 16), 17), 18) Bob's UA entity responds with the 200 OK message to the CANCEL request. S-CSCF P-CSCF Carol SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP INVITE (SDP Alice) SIP 180 Ringing SIP 180 Ringing SIP 180 Ringing SIP 180 Ringing SIP 180 Ringing SIP 181 Call Is Being Forwarded SIP 181 SIP CANCEL SIP CANCEL SIP CANCEL SIP 200 OK SIP 200 OK SIP 200 OK SIP 487 Request Terminated SIP 487 Request Terminated SIP 487 Request Terminated SIP ACK SIP ACK SIP ACK Figure 5.5. CFNR VoLTE and ViLTE 19), 20), 21) Bob's UA entity responds with the message 487 Request Terminated to the INVITE request. 22), 23), 24) The A