A receiving device in a trunked radio communications system determines transmit and receive frequencies by a mapping. The receiving device receives a voice channel number, wherein the voice channel number is communicated in an outbound signaling message defined by Motorola's 3600-baud radio trunking protocol. The receiving device tunes to about a transmit frequency of 806.0250 MHz, if the voice channel number is 440; tunes to about a transmit frequency of 806.0500 MHz, if the voice channel number is 441; tunes to about a transmit frequency of 806.0750 MHz, if the voice channel number is 442; etc. The receiving devices tunes to about a receive frequency of 852.5250 MHz, if the voice channel number is 500; tunes to about a receive frequency of 852.5500 MHz, if the voice channel number is 501; tunes to about a receive frequency of 852.5750 MHz, if the voice channel number is 502; etc.