Thinking about this subject a little more, another option could be Teamspeak...
Apps for Android and IOS are available giving direct access to talk group from a mobile device, including support for PTT button.
You can even buy Bluetooth speaker/mic with PTT for the like of Zello, to give that "real radio" feel hi
Servers can also be run on a raspberry pi making a gateway to a radio as another option.
One item that will need consideration with any system, is a need for a main calling channel, with the option to QSY for one-to-one working OR be able to private call a station to keep the main channel open.
73s Gary M1EGI