With regard to status signalling via button, there is some pioneering use of MDTs for that.
I might be a dinosaur because of being in the business 36 years, but doing things the "old way" has its advantages. If status button signalling was the only way I knew of who is responding, that means I'd have to look at my MDT to see that. For every second I'm looking at my MDT, that's a second I'm not looking at vehicular traffic.
Also if I've arrived on the scene, I'm out of the vehicle and thus away from the MDT. So the only way I know of other unit arrivals is hearing that, because I can't see that on the MDT.
Having said all that, status signalling can be advantageous. Currently our system is that when Truck A calls "Responding" the dispatcher repeats "Truck A Responding". If we did have status signalling, Truck A could press its "Responding" button saying nothing verbally. The dispatcher sees that Truck A is responding, all he or she would do is say that on the radio. So instead of two radio transmissions, all there would be is one. That can be extremely helpful because now there could be several units trying to call responding all at the same time. No one is heard.