A service provisioning mediator entity in a communications system is provided with information defining a set of service provisioning triggering messages. In response to receiving a service provisioning triggering message including an identifier associated with a user from a further network entity involved in providing the service to the user, the service provisioning mediator entity triggers service provisioning in the communications system for the user. The service provisioning triggering message may be sent by a controlling network entity upon receiving from an information store a message indicating that a user attempting to register is not recognized. If the user is recognized, trigger information in the information store may be used for directed a session to a network entity configured to handle service provisioning triggering messages. As a further alternative, a user may request service provisioning by sending a service provisioning triggering message from a communication device.