A method and apparatus for enabling delivery of a message in a communication system comprising an IMS domain and a circuit switched domain. Upon receiving a registration message in the IMS domain from a user equipment operated by a user, wherein the message comprises an Equipment Identifier valid for the user equipment and an IMPI valid for the user, the Equipment Identifier and the IMPI are extracted from the received message. Then a creation or an update of a mapping of the Equipment Identifier to the IMPI and a C-MSISDN is initiated. Upon receiving a message, relating to the user, which is to be communicate between the IMS and circuit switched domains in the communication system, a use of the mapping for translating between the Equipment Identifier, IMPI and the C-MSISDN is initiated.