he case of ISUP interconnection: - it transfers to the IMS-MGW gateway the SDP message associated with the INVITE request; - it commands the constitution of the context and the end-points at the level of the IMS-MGW gateway; - it retrieves the characteristics of the media flow of the interface on the IP side and on the TDM side of the IMS-MGW gateway. In the case of a BICC interconnection, the configuration of the IMS- MGW gateway can for example correspond to a transcoding of voice: adaptive multi-rate/narrow band (AMR/NB) codec, at the 4G network side; - half rate (HR) or full rate (FR) codec, at 2G network side. 6), 7) The MGCF entity transmits the response 183 Session Progress to the INVITE request in which the SDP messages recovered from the IMS-MGW gateway is inserted. 8), 9) The subsequent PRACK request acknowledges of the provisional response 183 Session Progress. 10), 11) The 200 OK message is the response to the PRACK request. 12), 13) The confirmation of the resource reservation on the 4G network is indicated to the MGCF entity in an SDP offer contained in the UPDATE request. 14) The MGCF entity transmits to the CS network the ISUP/BICC COT to inform him that the resource is available on the caller's side. 15), 16) The 200 OK message is the response to the UPDATE request. VoLTE and ViLTE 17) The network operating in CS mode reserves the resources and responds with an ISUP ACM message indicating that the call has been presented to the callee. 18), 19) The MGCF entity transmits the provisional response 180 Ringing to the INVITE request to trigger the ring back tone at Alice's UA entity. 20) When the callee hangs up, the MGCF entity receives the ISUP/BICC ANM message. The MGCF entity connects the end-points at the level of the IMS-MGW gateway. 21), 22) The MGCF entity sends the definitive response 200 OK to Alice's UA entity for stopping the ring back tone. 23), 24) The 200 OK response is acknowledged by the ACK request. The procedure for establishing the session initiated by the IMS network for SIP-I sig