A method of registering users in a mobile communications system, the method comprising the steps of: storing in a user information store primary and secondary identifiers identifying a first serving controller; receiving a request from a serving controller, said request identifying that serving controller using primary and secondary identifiers; comparing the primary identifier in the user information store with the primary identifier in said request and, if there is a mismatch, comparing the secondary identifier in the user information store with the secondary identifier in said request wherein, when there is a mismatch between the secondary identifiers it is determined that the serving controller identified in the request is not the first serving controller; and when there is a match between the secondary identifiers it is determined that the serving controller identified in the request is the first serving controller.