A system for providing advanced voice services in at least one wireless communications network, the system comprising: one or more servers (108, 110, 112, 114, 116, 118, 120, 122) that interface to the at least one wireless communications network to perform advanced voice services for a plurality of mobile units in the at least one wireless communications network, wherein the advanced voice services include an instant two-way half-duplex voice call within a group of the mobile units comprising a Push-to-Talk-over-Cellular, PoC, call session; wherein the one or more servers (108, 110, 112, 114, 116, 118, 120, 122) and the plurality of mobile units communicate with each other using control messages within the at least one wireless communications network, and at least one of the one or more servers (108, 110, 112, 114, 116, 118, 120, 122) switches voice messages for the advanced voice services between the plurality of mobile units across the wireless communications network; wherein at least one of the one or more servers (108, 110, 112, 114, 116, 118, 120, 122) manages the PoC call session by acting as an arbitrator for the PoC call session and controls the sending of the control messages and the voice messages to and from the plurality of mobile units; and wherein at least one of the one or more servers (108, 110, 112, 114, 116, 118, 120, 122) performs unicast and multicast delivery of the control messages and the voice messages to and from the mobile units, wherein a unicast or multicast delivery method is selected for each of the plurality of mobile units based on a respective access network type for each of the plurality of mobile units. one or more servers (108, 110, 112, 114, 116, 118, 120, 122) that interface to the at least one wireless communications network to perform advanced voice services for a plurality of mobile units in the at least one wireless communications network, wherein the advanced voice services include an instant two-way half-duplex voice call within a group of the mobile units comprising a Push-to-Talk-over-Cellular, PoC, call session; wherein the one or more servers (108, 110, 112, 114, 116, 118, 120, 122) and the plurality of mobile units communicate with each other using control messages within the at least one wireless communications network, and at least one of the one or more servers (108, 110, 112, 114, 116, 118, 120, 122) switches voice messages for the advanced voice services between the plurality of mobile units across the wireless communications network; wherein at least one of the one or more servers (108, 110, 112, 114, 116, 118, 120, 122) manages the PoC call session by acting as an arbitrator for the PoC call session and controls the sending of the control messages and the voice messages to and from the plurality of mobile units; and wherein at least one of the one or more servers (108, 110, 112, 114, 116, 118, 120, 122) performs unicast and multicast delivery of the control messages and the voice messages to and from the mobile units, wherein a unicast or multicast delivery method is selected for each of the plurality of mobile units based on a respective access network type for each of the plurality of mobile units.