S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-7f. SIP 180 Ringing response (INVITE):
from P-CSCF1 to UE1
(45 of 70)
: P-CSCF1 rewrites the Record-Route header to add the comp=sigcomp parameter to its own SIP URI and its port number negotiated during the security agreement.