Apparatus configured to implement a Proxy Call Session Control Function for use within an IP Multimedia Subsystem core network. The apparatus comprises a receiver for receiving a session initiation request in respect of an IP Multimedia Subsystem service, and a threshold determiner for determining a threshold applicable to user traffic associated with said service. The apparatus further comprises a sender for sending a session authorization request to a policy decision function, the request including said threshold and a service data flow identifier identifying the user plane flow for said session.