I used a version of it on UK Vodalone company phones in 2005/6. The people who wanted PTT turned it on and nominated up to 3 numbers, then it worked like a normal walkie talkie. Advantage was that blokes pulling cables in on site A could do so without bothering about people working a crane on site B. My phone didn't have a PTT button, so it was voice-operated.
$MEGACORP now uses O2, and no such thing is possible.
PMR radios have rather poor range, and Icom F3000 are expensive. PTT mobiles are a clever idea.