An interface (52) is provided between a satellite telephone system (4) and a cellular telephone system (5) for allowing calls to a user's cellular telephone to be diverted to a satellite telephone handset (25) when the user is unable to use his cellular telephone, for example when on board an aircraft (2) fitted with a satellite telephone system (20). The user inserts a card (which may be the SIM of his mobile phone, or a card compatible with existing satellite telephone equipment) into a suitable reader in the satellite telephone (20), which causes the satellite ground station's card verification system (42) to connect to a host cellular network (50). The host cellular network has an interface unit (52) which emulates the operation of a normal base station, so that the host cellular system (50) acts as if the mobile user's terminal is roaming on the host network, but is currently "busy". The interface (52) also resets the user's existing call diversion instructions so that when the host cellular system (5) receives a busy tone from the base station emulator (52), it routes calls to the directory number of the terminal (20). Data messages originally formatted for transmission over the cellular telecommunications network for generation of a display on a display unit of a cellular telephone, can instead be transmitted over the satellite network for display on the information and entertainment system of the aircraft by replacing the address information in the original data message by address information appropriate to the selected display unit to generate an amended data message. A new data message is created, addressed to the server of the information display system and having the amended data message as payload. The onboard server has means for converting the amended data message into control instructions suitable to display the information content of the data message on the information display unit identified in the amended data message.