A system for providing advanced voice services in at least one wireless communications network (124, 126, 142), comprising: one or more servers that interface to the wireless communications network (124, 126, 142) to perform advanced voice services for one or more 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 Push-to-Talk-over-Cellular, PoC, call session; wherein 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 (124, 126, 142), and at least one of the servers switches voice messages for the advanced group services between the mobile units across the wireless communications network (124, 126, 142); wherein at least one of the servers 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 mobile units; and wherein at least one of the servers performs an effective presence delivery method that reduces presence notifications delivered to the mobile units participating in the advanced voice services, wherein the effective presence delivery method comprises: creating a list identifying watchers served by a same cell or a same group of cells in the wireless communications network (124, 126, 142); sending first presence notifications to no more than a first specified number of the watchers served by the same cell or the same group of cells during a first specified time period; throttling second presence notifications at the server when a number of the watchers served by the same cell or the same group of cells is greater than the first specified number so that a total number of presence notifications delivered in the same cell or the same group of cells is not more than a second specified number during the first specified time period. one or more servers that interface to the wireless communications network (124, 126, 142) to perform advanced voice services for one or more 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 Push-to-Talk-over-Cellular, PoC, call session; wherein 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 (124, 126, 142), and at least one of the servers switches voice messages for the advanced group services between the mobile units across the wireless communications network (124, 126, 142); wherein at least one of the servers 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 mobile units; and wherein at least one of the servers performs an effective presence delivery method that reduces presence notifications delivered to the mobile units participating in the advanced voice services, wherein the effective presence delivery method comprises: creating a list identifying watchers served by a same cell or a same group of cells in the wireless communications network (124, 126, 142); sending first presence notifications to no more than a first specified number of the watchers served by the same cell or the same group of cells during a first specified time period; throttling second presence notifications at the server when a number of the watchers served by the same cell or the same group of cells is greater than the first specified number so that a total number of presence notifications delivered in the same cell or the same group of cells is not more than a second specified number during the first specified time period. creating a list identifying watchers served by a same cell or a same group of cells in the wireless communications network (124, 126, 142); sending first presence notifications to no more than a first specified number of the watchers served by the same cell or the same group of cells during a first specified time period; throttling second presence notifications at the server when a number of the watchers served by the same cell or the same group of cells is greater than the first specified number so that a total number of presence notifications delivered in the same cell or the same group of cells is not more than a second specified number during the first specified time period.