S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-11a. SIP ACK request (UE1 to UE2):
from UE1 to P-CSCF1
(66 of 70)
UE1 starts the media flow for this session, and responds to the 200 (OK) response with an ACK request sent to P-CSCF1.