es Alice's UA entity to retrieve the IP addresses of the CSCF entities that need to process the subsequent requests. Bob's UA entity indicates the value 100rel in the header Require to indicate that the response requires acknowledgement from Alice's UA entity. To correlate the acknowledgement with the response, Bob's UA entity inserts the header RSeq. In the response 183 Session Progress, Bob's UA entity gives an SDP response in which it chooses a type of codec from those proposed by Alice's UA entity. Bob's UA entity indicates in the SDP message of the 183 Session Progress response that resource reservation is also necessary on its part before establishing a session. 12) to 21) On receiving of the 183 Session Progress response, the P-CSCF entity initiates the establishment of dedicated bearer. They are identical to messages 9 to 18 described in the preceding paragraph. 22), 23), 24) The 183 Session Progress response is transmitted to the domain homeA. net. 25), 26), 27) Subsequent PRACK request is received from the domain homeA. net, indicating that Alice's UA entity has acknowledged receiving 183 Session Progress response. 28), 29), 30) The 200 OK message is the response to the PRACK request. 31), 32), 33) Subsequent UPDATE request is received from the domain homeA. net, indicating that the dedicated bearer on the Alice's side is established. 34), 35), 36) The 200 OK message is the response to the UPDATE request, indicating that the dedicated bearer on the Bob's side is established. VoLTE and ViLTE 37) to 40) The 180 Ringing response to the initial INVITE request is sent to the domain homeA. net, indicating that the Bob's phone is ringing. 41) The 200 OK response to the initial INVITE request indicates that Bob picks up his phone. 42) The P-CSCF entity sends the PCRF entity the DIAMETER AAR message to indicate that Bob took the call. 43) The PCRF entity transmits to the PGW entity the DIAMETER RAR message to unblock the dedicated bearer. 44) The PGW entity responds to the PCRF entity with the DIAMETER AAR messa