I'm sure someone mentioned it, but you can use the physical hardware keys to do what you want to do, in the event you find yourself in a situation with those two soft buttons missing.

While in a call, you can press the phone hardware key to bring up the dialer pad. ...and you can end a call with the "End" hardware key.

...even for things like scrolling to the bottom of lists that are cut off or selecting items when the "ok" button isn't accessible, you can use the hardware buttons. Center button is an "ok" button.