A method and apparatus for registering a mobile node such as a User Equipment (UE) of a UMTS telecommunications network with a subsystem of the network such as an IP Multimedia Subsystem (IMS). A Policy and Charging Rules Function (PCRF) node provides policy and charging rules, and a Gateway GPRS Support Node (GGSN) enforces the rules for traffic flows. The UE sends a registration request to the GGSN, which selects a PCRF. When a Proxy Call Session Control Function (P-CSCF) receives the registration request, the P-CSCF selects a candidate PCRF and validates the selection. If the candidate PCRF is the PCRF selected by the GGSN, the registration is successful. If not, the P-CSCF may select another PCRF and repeat the validation process.