A digital communication system comprises receiving and transmitting apparatus at each end of a communication link. A call is set up from either end of the link by first transmitting a paging signal to initiate the link and to identify the address of the intended recipient equipment. A paging signal has a unique carrier modulation envelope and comprises packets of data interleaved with periods of unmodulated carrier such that the presence of the paging signal may be identified by a suitable modulation envelope detector (22, 24, 26) without recourse to decoding the data contained within the paging signal.