"The Voxer application, for example, is capable of supporting full-duplex communication, allowing two (or more) participants of a conversation to engage in near real-time communication, similar to a synchronous telephone call. Participants are therefore required to select the PTT feature for the entire duration of the full-duplex exchange, which may be inconvenient after a certain period of time."
http://www.google.com/patents/US20130295982