1980s. Out-of-band individual calling Trunked radio systems have built-in unit ID and selective calling features. Each trunked system has its own unique features. See the article for a specific system to learn more. Two-way radio systems using digital modulation schemes such as TDMA can embed unit ID and selective calling into the data stream multiplexed in parallel with the voice. See the article pertaining to a specific system to learn more. References Radio technology