A wireless communication system (200) for providing two-way radio communication includes a base station (340) comprising a receiver (352) and a transmitter (354). The receiver receives transmissions from a first mobile communication unit (210) on a first frequency, assigned for reception. The transmitter transmits on a second frequency, assigned for transmission. The base station (340) is operable to receive a preemption call from a second mobile communication unit (220) on the first frequency, when the first frequency is not in use for reception and the second frequency is in use for transmission. Otherwise the base station (340) receives a preemption call on the second frequency. The base station (340) may comprise a second receiver (344), operable to receive the preemption call on the second frequency during a planned interruption to transmissions from the transmitter (354). Alternatively, the receiver (352) may receive the preemption call on the second frequency, during the transmission interruption.