P-CSCF
Proxy – Call Session Control Function

The P-CSCF acts as the ingress and egress point to and from a service provider’s IMS domain with respect to the IMS client. The P-CSCF has a large number of responsibilities, including: onward routing of registration and session requests to the correct nodes in the network, ensuring the S-CSCF (Serving CSCF) is kept updated on the access network the subscriber is using, providing session information to the PCRF (Policy and Charging Rules Function) and maintaining a secure connection with the client device.