Communication units, wireless communication system and method of communication therein

A wireless subscriber communication unit (300) is capable of communication on a direct mode communication system (100). The wireless subscriber communication unit (300) comprises a receiver and a signal processor (308), operably coupled to the receiver, for processing a received message relayed from an intermediary communication unit (150). The signal processor (308) comprises a group call processing function (340) configured to identify from the received message one or more call groups supported by the intermediary communication unit (150). An intermediary communication unit, a method of communicating, a direct mode communication system and a radio frequency transmission message are also provided. In this manner, by provision of a mechanism to allow an intermediary communication unit such as a gateway or repeater to indicate the DMO call groups that it supports, the efficiency of using group calls in, for example, a TETRA DMO communication system is improved.