A multimedia gateway that has an IP Multimedia Subsystem Subscriber Identity Module (ISIM), a first interface to communicate with a client terminal, and a second interface to communicate over a Gm interface with Call Session Control Function (CSCF) and to communicate over a Ua interface with an IMS application server (IMS AS) that acts as a Generic Bootstrapping Architecture Network Application Function located in an IMS network, comprising: registering means for assigning an IMS Public Identity (IMPU) obtained from the ISIM to the client terminal in response to a SIP registration request message received from the client terminal, and sending the SIP registration request message over the Gm interface for registering the IMPU assigned to the client terminal with the IMS network; request receiving means for receiving, from the client terminal, an application protocol request message that specifies a destination IMS AS acting as GBA NAF and an application protocol, and identifying the IMPU assigned to the client terminal; establishing means for establishing a TLS session for communication with the IMS AS acting as GBA NAF by means of the application protocol, and establishing a TLS connection with the IMS AS acting as GBA NAF over the TLS session; authentication information sending means for sending authentication information derived from a NAF-specific key to the said IMS AS acting as GBA NAF derived from the ISIM that contains the identified IMPU to the IMS AS acting as GBA NAF; request sending means for sending the application protocol request message together with the identified IMPU to the IMS AS acting as GBA NAF through the TLS connection over the Ua interface; response receiving means for receiving an application protocol response message from the IMS AS acting as GBA NAF through the TLS connection as a response to the application protocol request message; and response sending means for sending the application protocol response message to the client terminal. registering means for assigning an IMS Public Identity (IMPU) obtained from the ISIM to the client terminal in response to a SIP registration request message received from the client terminal, and sending the SIP registration request message over the Gm interface for registering the IMPU assigned to the client terminal with the IMS network; request receiving means for receiving, from the client terminal, an application protocol request message that specifies a destination IMS AS acting as GBA NAF and an application protocol, and identifying the IMPU assigned to the client terminal; establishing means for establishing a TLS session for communication with the IMS AS acting as GBA NAF by means of the application protocol, and establishing a TLS connection with the IMS AS acting as GBA NAF over the TLS session; authentication information sending means for sending authentication information derived from a NAF-specific key to the said IMS AS acting as GBA NAF derived from the ISIM that contains the identified IMPU to the IMS AS acting as GBA NAF; request sending means for sending the application protocol request message together with the identified IMPU to the IMS AS acting as GBA NAF through the TLS connection over the Ua interface; response receiving means for receiving an application protocol response message from the IMS AS acting as GBA NAF through the TLS connection as a response to the application protocol request message; and response sending means for sending the application protocol response message to the client terminal.