A wireless telecommunications device configured to use Session Initiation Protocol in communication with other telecommunications devices comprises a modem subsystem configured to deploy Internet Protocol Multimedia Subsystem services using Session Initiation Protocol and comprising a Session Initiation Protocol stack; and an application processing engine configured to deploy Internet Protocol Multimedia Subsystem services using Session Initiation Protocol and comprising a Session Initiation Protocol stack. The device comprises a logical element configured to capture Session Initiation Protocol messages from Internet Protocol Multimedia Subsystem services to an external Session Initiation Protocol proxy server and to function as a Session Initiation Protocol Back-to-Back User Agent; and send to the external Session Initiation Protocol proxy server Session Initiation Protocol messages based on the captured messages. The need for special Application Programming Interface between the two subsystems is avoided, and multiple SIP request messages need no longer be sent.