1. A method for selecting a serving call session control function unit within a service communication system based on the “Internet Protocol Multimedia Subsystem” in order to register a user of a mobile communication terminal for at least one communication service within the mobile communication system, comprising: providing a plurality of serving call session control function units for the IMS-based service communication system; providing at least one interrogation call session control function unit for the IMS-based service communication system; providing at least one policy decision point unit for the mobile communication system; assigning a user context server unit to a policy decision point unit; determining user information using the user context server unit; determining network information using a network context server unit; interlinking the user information and the network information in the policy decision point unit; deriving, by the policy decision point unit, user-specific or network-specific selection criteria from said user and network information taking the user and network information into consideration; and selecting a suitable S-CSCF unit by considering the derived user-specific or network-specific selection criteria.