A system and method for optimizing large group calling using unicast and multicast transport bearers for Push-to-talk-over-Cellular (PoC). One or more servers interface to a wireless communications network to perform advanced voice services for mobile units therein, including the PoC call session. The servers and mobile units communicate with each other using control messages, and the servers switch voice messages between mobile units. At least one of the servers manages the PoC call session by acting as an arbitrator, and controls sending of control messages and voice messages to and from mobile units. The server performs unicast and multicast delivery of control messages and voice messages to and from mobile units, thereby enabling large group calling involving a large number of participants with optimized use of network and server resources.
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