hone terminal connected to a CS network. The procedure for establishing the session initiated by the IMS network for ISUP or BICC telephone signaling is illustrated in Figure 6.5. 1) The call is generated by Alice's UA entity, by way of an INVITE request whose identity TEL URI contains the telephone number of the callee. The S-CSCF entity carries out an ENUM resolution with the DNS server, to convert the TEL URI into a SIP URI. The INVITE message contains the session description protocol (SDP) message of Alice's UA entity. As the callee is not a client of the IMS network, the response from the DNS server is negative and the S-CSCF entity then routes the INVITE request to the BGCF entity. VoLTE and ViLTE S-CSCF network SIP INVITE (SDP Alice) SIP 100 Trying SIP INVITE (SDP Alice) SIP 100 Trying ISUP/BICC IAM SIP 183 Session in Progress (SDP IMS-MW) SIP 183 Session in Progress (SDP IMS-MW) SIP PRACK SIP PRACK SIP 200 OK SIP 200 OK SIP UPDATE (SDP Alice) SIP UPDATE (SDP Alice) ISUP / BICC COT SIP 200 OK (SDP IMS-GW) SIP 200 OK (SDP IMS GW) ISUP / BICC ACM SIP 180 Ringing SIP 180 Ringing ISUP / BICC ANM SIP 200 OK SIP 200 OK SIP ACK SIP ACK Figure 6.5. Session establishment initiated by IMS network ISUP or BICC signaling 2) The BGCF entity responds to the S-CSCF entity with the 100 Trying message that allows the blocking of the retransmission timer of the INVITE request. 3) The BGCF entity determines the MGCF entity which interconnects with the CS network, either from an ENUM resolution, or from internal correspondence tables, and forwards the INVITE request. 4) The MGCF entity responds to the BGCF entity with the 100 Trying message that allows the blocking of the retransmission timer of the INVITE request. Interconnections 5) The MGCF entity converts the INVITE request into an ISUP/BICC IAM message. This message is transmitted to the IMS-MGW using SIGTRAN transport, and then to the network in CS mode. When the MGCF entity receives the INVITE request, it performs the following operations with the IMS-MGW gateway, in t