My read is that "Express Chat" is effectively a push-to-talk feature. It's implemented as an Android and iOS app: <https://play.google.com/store/apps/details?id=com.moon.expre...> (Android) and <https://apps.apple.com/in/app/express-chat/id6745220803> (iOS).

Several carriers offer this, usually aimed at corporate users (construction, maintenance, fleet workers, emergency responders). See Kyocera and Sonim, particularly their rugged phone offerings (Dura from Kyocera, XP from Sonim).

Carrier-based PTT is typically limited only to other phonesets on that carrier, though in practice the feature is generally accessed through a programmable key, and can be configured to use another (unlimited) press-to-talk app, such as Zello (which I've seen before) or Express Chat (#TIL).