The PSTN establishes a bearer path to the MGW, and signals to the MGCF with a IAM message, giving the trunk identity and destination information.
The MGCF initiates a H.248 command, to seize the trunk and an IP port.
The MGCF initiates a SIP INVITE request addressed to a tel URI or, if directed by operator's local policy, to a SIP URI (using an E.164 address in the user portion and the setting user=phone), includes an initial SDP in the INVITE request, and forwards the request to a configured I-CSCF, as per the proper S-S procedure. If configured through policies, the MGCF adds to the SIP INVITE attestation information based on the trunk identity or other sources of the request.
The media stream capabilities of the destination are returned along the signalling path, per the S-S procedures.
MGCF initiates a H.248 command to modify the connection parameters and instruct the MGW to reserve the resources needed for the session.
MGCF decides the offered set of media streams for this session, confirms receipt of the Offer Response and sends the Response Confirmation per the S-S procedures.
Terminating end point responds to the Response Confirmation. If Optional SDP is contained in the Response Confirmation, the Confirmation Acknowledge will also contain an SDP response.
MGW reserves the resources needed for the session.
When the resource reservation is completed, MGCF sends the successful Resource Reservation message to the terminating endpoint, per the S-S procedures.
Terminating end point responds to the successful media resource reservation.
The destination endpoint may optionally perform alerting. If so, it signals this to the originating party by a provisional response indicating Ringing. This message is sent to MGCF per the S-S procedure.
If alerting is being performed, the MGCF forwards an ACM message to PSTN.
When the destination party answers, the terminating and S-S procedures result in a SIP 200-OK final response being sent to MGCF.
MGCF forwards an ANM message to the PSTN.
MGCF initiates a H.248 command to alter the connection at MGW to make it bi-directional.
MGCF acknowledges the SIP final response with a SIP ACK message.