Abstract: An on-site communication system includes a system for identifying communication receivers including acknowledge back capability and operating within the on-site communication system, and for controlling the delivery of messages to the communication receivers. The identification system operates on the same frequency as the on-site communication system, and includes an RF shielded enclosure for momentarily shielding the communication receivers from messsages transmitted on the on-site communication system. The RF shielded enclosure provides interference free identification of the communication receivers as the communication receivers pass through the enclosure, and enables the determination of those communication receivers which are operating within the coverage area of the on-site communication system, and those communication receivers which have left the coverage area of the on-site communication system.