Mobile Phone PTT

Push-to-talk cellular calls similarly provide half-duplex communications — while one person transmits, the other(s) receive. This combines the operational advantages of PTT with the interference resistance and other virtues of mobile phones.
Traditional mobile phone networks and devices utilize full-duplex communications, allowing customers to call other persons on a mobile or land-line network and be able to simultaneously talk and hear the other party. Such communications require a connection to be started by dialling a phone number and the other party answering the call, and the connection remains active until either party ends the call or the connection is dropped due to signal loss or a network outage. Therefore, the telephone communication protocol does not allow for casual and immediate transmissions to be sent to other parties on the network. While telephone calls require the lengthy process of dialling, network switching and routing, call setup, and waiting for the other party to answer, a two-way radio has a much quicker protocol because of the immediacy of push to talk communication.
Later versions of PTT are based on 2.5G or 3G packet-switched networks and use SIP and RTP protocols. These particular versions of PTT are called Push to Talk over Cellular, which is abbreviated PoC. When used with GSM and CDMA networks, the PTT service commonly does not use up the regular airtime minutes that are available for general voice calls.

About Push To Talk (PTT)

Push-to-talk (PTT), also known as Press-to-Transmit, is a method of conversing on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode.

CB radio with push-to-talk microphone switch

For example, an air traffic controller usually talks on one radio frequency to all aircraft under his supervision. All can hear each other's transmissions and those of the controller, and take turns speaking, using procedure words such as "over" and "out". Thus they are aware of each other's actions and intentions, and do not hear any noise from the ones who are not speaking. Similar considerations apply to police radio, the use of business band radios on construction sites, and other situations requiring coordination of several parties. Citizens Band is another example of classic push to talk operation.
Where a user may be too busy to handle a talk switch, for example in combat aircraft, voice operated switches are sometimes employed. Some systems use PTT ID to identify the speaker.
Push to Talk over Cellular (PoC) is a service option for a cellular phone network which permits subscribers to use their phone as a walkie-talkie with unlimited range. A typical Push to Talk connection connects almost instantly. One significant advantage of PoC / PTT is that it allows a single person to reach an active talk group with a single button press; users need not make several calls to coordinate with a group.