PTT (Push-To-Talk) - definition
A two-way communication service which works like a "walkie-talkie".
PTT is half-duplex so communication can only be transferred in one direction at a time in contrast to mobile phone calls which are full-duplex and both parties can hear each other.
PTT requires the person speaking to press a certain button while talking and to release it when they are done so the listener can answer by repeating the same steps. In this way people control which one can speak and be heard and the system knows in which direction the information should be transferred.
Most PTT systems allow group calls (one person speaks to all the members of the group).
The newest PTT systems use VoIP technology and the signal is transferred over the 2.5G or 3G network.