P-CSCF
Proxy Call Session Control Function: the IMS entry point for SIP signaling from the UE, providing SIP proxy, IPsec security association, and emergency call detection.
The P-CSCF is the IMS box the phone talks to first. Every SIP message from the UE — registration, call setup, the lot — enters and leaves IMS through it, which makes it the natural place to put the things that have to happen right at the trust boundary. It sets up the IPsec security association with the UE so SIP signalling is protected over the access network, it can police the media and apply QoS, and it detects emergency calls so they get routed to a proper emergency path rather than the normal one.
A practical point: the UE discovers its P-CSCF during PDP/PDU session setup (often via a DHCP option or the protocol configuration options in the bearer signalling), and it stays the anchor proxy for that registration. Although "P" stands for Proxy, it doesn't decide where calls go — it forwards toward the I-CSCF/S-CSCF in the home network. It's the policed front door of IMS; the routing brains sit deeper in.
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