S4. IMS Session Initiation signalling flow —
mobile origination in home network
termination in CS network
S4-1b. SIP INVITE request (UE1 to UE2):
from P-CSCF1 to S-CSCF1
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S-CSCF#1 validates the service profile of this subscriber and evaluates the initial filter criterias. For this example, assume no Application Server involvement.
S-CSCF#1 performs an analysis of the destination address, and determines the destination is on the PSTN. S-CSCF forwards the INVITE request to the BGCF in the local network.