PTT is the traditional radio communications abbreviation for Push-To-Talk. Nextel tried to trademark it a few years ago, but they didn't get very far.

Since Nextel, and the other cell phone based 2-way services operate similarly to 'trunked' digital 2-way radios (such as are used by emergency services in many areas these days), you have to hold the PTT button down until you hear the go ahead signal (in Nextel's case, the triple beep, which sound they *have* trademarked) before you can talk.