An architecture, client specification and application programming interface (API) for supporting advanced voice services (AVS) for use in handsets in order to support advanced voice services (AVS) for wireless communications systems. The handset or mobile unit executes a client application therein for performing the call setup and in-band signaling with the wireless network for the group voice services, and executes a presence/group management application therein for performing presence and group management functions related to the group voice services in the mobile unit.