S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-2e. SIP 183 Session Progress response:
from S-CSCF1 to P-CSCF1
(13 of 70)
P-CSCF1 authorizes the resources necessary for this session. The approval of QoS commitment either happens at this stage or after 200 OK of INVITE based on operator local policy.