A satellite communications system provides a communications service to a mobile terminal on which different communications applications may be run. Calls are set up between any of the applications via a satellite to a network management center which provides different service adaptors which adapt the calls to different types of service provided over terrestrial networks such as telephony, facsimile, internet or ATM services. The bandwidth allocated to each call over the satellite link may be varied during the call according to demand either from the relevant application or from the network management center. Multiple calls may be connected concurrently to or from different applications running on the mobile terminal. A maximum bandwidth is allocated to each call. Efficient use is thereby made of the limited bandwidth available over the satellite, according to the instantaneous bandwidth requirements of different applications. For real-time calls requiring multiple slots per frame in a TDMA channel, the slots are mutually spaced apart to reduce delay.