The IP Multimedia Subsystem relies on three Call Session Control Functions to manage every VoLTE call. The P-CSCF (Proxy CSCF) is the first contact point for the UE, acting as an outbound SIP proxy that enforces security through IPsec and requests dedicated bearers via the Rx interface to the PCRF. The I-CSCF (Interrogating CSCF) sits at the network boundary and queries the HSS to determine which S-CSCF should handle a particular subscriber. The S-CSCF (Serving CSCF) performs the core session control logic.