In FDMA communications between two or more primary radios (41 42; 141 142) use is made of a quiescent period in the primary communication. The quiescent period is used by secondary radios (43 44; 143 144) to communicate with each other. The quiescent period may be a transmit interrupt (TI) frame normally provided in a DIIS protocol in order to permit return signalling from a called radio (43 44). The secondary radios (43 44) may signal to each other during the TI frame. Alternatively the quiescent period may be a gap in channel activity in which case regulated access telegrams (96) are transmitted by either a calling primary device (141) or a last to call primary device (141) or a base station (141) the regulated access telegrams determining a reservation window period during which secondary devices (143 144) may signal to one another.