A system for providing advanced voice services in a wireless communications network (124, 126, 142), comprising: one or more servers that interfaces to the wireless communications network (124, 126, 142) to perform advanced voice services for a plurality of 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, 42), 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 sending of the control messages and the voice messages to and from the mobile units; and wherein at least one of the servers performs a presence delivery method comprising: splitting call history data of a mobile unit across different periods of time, wherein the mobile unit is one of the plurality of mobile units; determining a subset of contacts in the call history data for each of the different periods of time; consolidating the contacts in each of the subset of contacts produce a relevant contact list; and limiting real-time delivery of presence notifications for the mobile unit to a subset of presence notifications pertaining to the mobile unit according to the relevant contact list. one or more servers that interfaces to the wireless communications network (124, 126, 142) to perform advanced voice services for a plurality of 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, 42), 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 sending of the control messages and the voice messages to and from the mobile units; and wherein at least one of the servers performs a presence delivery method comprising: splitting call history data of a mobile unit across different periods of time, wherein the mobile unit is one of the plurality of mobile units; determining a subset of contacts in the call history data for each of the different periods of time; consolidating the contacts in each of the subset of contacts produce a relevant contact list; and limiting real-time delivery of presence notifications for the mobile unit to a subset of presence notifications pertaining to the mobile unit according to the relevant contact list. splitting call history data of a mobile unit across different periods of time, wherein the mobile unit is one of the plurality of mobile units; determining a subset of contacts in the call history data for each of the different periods of time; consolidating the contacts in each of the subset of contacts produce a relevant contact list; and limiting real-time delivery of presence notifications for the mobile unit to a subset of presence notifications pertaining to the mobile unit according to the relevant contact list.