S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-8a. SIP PRACK request (UE1 to UE2):
from UE1 to P-CSCF1
(47 of 70)
: takes the value of the Contact header of the received 180 Ringing response.
: contains the security agreement as represented by the received Security-Server header.