In the originating network, the I-CSCF receives the SIP Registration message from the P-CSCF and forwards it to the appropriate S-CSCF. With the selection of the S-CSCF, the I-CSCF steps out of the signaling path and takes no additional actions on behalf of the UE.
When the I-CSCF is in the terminating network, it is actively involved in session establishment following the SIP Invite message. With respect to the terminating network, the I-CSCF acts as the IMS gateway for messages coming from the originating S-CSCF. The Invite request and the progress messages route through the terminating network’s I-CSCF.