S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-1d. SIP INVITE request (UE1 to UE2):
from I-CSCF2 to S-CSCF2
(5 of 70)
NOTE: I-CSCF2 does not add itself to the Record-Route header, as it has no need to remain in the signalling path once the session is established.
S-CSCF#2 validates the service profile of this subscriber and evaluates the initial filter criterias.