I-CSCF

The I-CSCF is a routing helper that sits at the edge of an IMS network and answers one question: which S-CSCF should handle this user? When a registration or an incoming session arrives from outside (or from the P-CSCF), the I-CSCF queries the HSS/UDM to find out either where the user is already registered or which S-CSCF to assign, then forwards the request there. It's largely stateless between requests — it looks things up and passes them on.

Two jobs make it worth having as a distinct node. First, it's the published entry point for the network, so it can perform topology hiding (historically via the THIG function) to keep your internal S-CSCF addressing private from peer networks. Second, it centralises the HSS lookup so the rest of the path doesn't each have to do it. On a SIP trace you'll see it briefly in the signalling chain — present at registration and at the start of an incoming call, then out of the picture once the S-CSCF takes over.