How’s your WiFi infrastructure? Assuming you have the area covered with WiFi you could go PoC (PushToTalk Over Cellular). There are dedicated handsets you can use for this for as low as USD$50 ea on Ali Express. Being that it’s just WiFi there are no licenses to worry about. Having a 2 way radio type handset makes it pretty familiar, buy you can very much use it with existing mobile handsets. Zello seems to be the de-facto standard for this but i believe other options exist.