Improved throughput is provided in a spacecraft TDMA cellular communications system by in-call signalling without the need for flag bits, while still allowing the in-call signaling to be differentiated from other traffic. The terrestrial locations include mobile user terminals and gateways which provide connections to the land telephone line system. The spacecraft includes a transmitter and receiver, and an antenna which forms a plurality of spot beam footprints, in which the terrestrial terminals are located. Each of the terrestrial terminals transmits signals to, and receives signals from, the spacecraft. Each of the terrestrial terminals includes an encoder/decoder, for encoding at least call maintenance control signals, for temporarily interrupting the traffic signals to allow another action. The control signals may have bit content which exceeds the bit-carrying capability of the TDMA bursts, so transmission of the control signals must extended over a plurality of bursts.