Proxy Call State Control Function (P-CSCF) performs the following functions:
is the first contact point for UE within IM CN subsystem, forwards the registration to the I-CSCF to find the S-CSCF and after that forwards the SIP messages between UE and I-CSCF/S-CSCF,
behaves as like a proxy in RFC 2543, i.e. accepts requests and services the internally or forwards them possibly after translation,
may behave also like a user agent (in RFC 2543), i.e. in abnormal conditions it may terminate and independently generate SIP transactions,
is discovered using DHCP during registration or the address is sent with PDP context activation,
may modify the URI of outgoing requests according to the local operator rules (e.g. perform number analysis, detect local service numbers),
detect and forward emergency calls to local S-CSCF,
generation of charging information,
maintains security association between itself and UE (User Equipment), also provides security towards S-CSCF,
provides the policy control function (PCF),
authorization of bearer resources, QoS management and Security issues are currently open in standardization.