A communication system that is particularly suited to switch a half duplex session to a full duplex session. The system includes a dispatch call controller that sets up a half duplex session; and a VoIP server that receives a request to switch the half duplex session to a full duplex session; forwards the request to the target subscriber unit; receives an acceptance of the request; and sets up the full duplex session between the subscriber units. In another embodiment, the dispatch call controller receives the request to switch to the full duplex session; forwards the request to the target subscriber unit; receives an acceptance of the request; and instructs the VoIP server to set up the full duplex session. In yet another embodiment, a service broker receives the request, instructs the VoIP server to set up the full duplex session, and instructs the dispatch call controller to terminate the half duplex session.