S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-1. SIP INVITE request (UE1 to UE2)
(1 of 70)
UE1 is located in a visited network, and determines P-CSCF1 via the CSCF discovery procedure.
P-CSCF1 knows the name/address of S-CSCF1 from a prior Registration performed by the UE.