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 signaling 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.