Push-to-Talk  (Press-to-Transmit, P2T, PTT) - These are half-duplex (one-way at a time) voice call connections over wireless networks in the manner made famous by "walkie talkies."

PTT PoC or Push to Talk over Cellular is a service option for a mobile phone network which allows subscribers to use their phones as walkie-talkies.
www.mobilein.com/push_to_talk.htm https://en.wikipedia.org/wiki/Push_to_talk