A system and method for relay-mode and direct-mode operations for Push-to-talk-over-Cellular (PoC) using WiFi technologies. One or more servers interface to a wireless communications network to perform advanced voice services for mobile units therein, wherein the advanced voice services include an instant two-way half-duplex voice call within a group of the mobile units comprising a PoC call session. Both the servers and the mobile units that use the advanced group services communicate with each other using control messages within the wireless communications network. At least one of the servers switches voice messages for the advanced group services between the mobile units across the wireless communications network. At least one of the servers manages the call session by acting as an arbitrator for the call session and by controlling the sending of the control messages and the voice messages to and from the mobile units. At least one of the mobile units performs Relay-Mode or Direct-Mode operations using WiFi communications for delivery of the control messages and the voice messages to and from others of the mobile units, thereby enabling the advanced voice services when access to the wireless communications network by the others of the mobile units is limited.