hey i think you have a bug in your keyborad and while calling
If I wanted to Call someone and I didn't have a cellular network and I pushed The cancel bottom The call wouldn't exist. The phone stuck in the GUI call
Without the cancellation bottom.
So I have to restart to fix it with the physical bottom it won't response I have
To do it with the software .
In the iOS 12 the same problem plus the keyboard is stuck too