For 911 calls, and most calls actually, they almost never show the screen of the phone being dialled anyway. They just show the actor tapping at the phone vaguely while the camera shows them from the side or the back with an intense expression, and then suddenly they hold it up to their ear and then they act like they're talking to 911.

And if for some reason they really do have to show the screen, they will just use a non-functional phone with a fake app or graphical effects in post-production, the same way they make ghosts and dragons appear and spaceships come crashing out of the sky, etc. A phone screen is a trivial effort to fake.