You are right that there is no definition of "button" in the Act, nor for that matter, has it been defined by case law. I think there is a valid argument for a 'virtual button' on a screen since very few phones now have physical buttons to place/end calls. From my experience though, the courts have allowed the pressing of an onscreen button, but do not allow swiping. So, if the person is simply pressing a virtual button to place/end the call, they should be ok. The problem however is that most folks don't know how to program their phones that way and need to swipe to get to the phone app, then want to dial by pressing the numbers on the screen, and THEN want to press the virtual button to place the call. That's definitely not what the exemption was intended for. It should only take a 1 time press of a button. Anything more than that and it becomes distracted driving; the whole point ("gravamen" as courts state) for the legislation.
Realistically though, I think most people can now use voice activation to place calls via their phone without touching any virtual buttons or at least should be able to program their phone so that it simply only takes 1 press of the 'virtual button' to place a call (i.e. a shortcut on the Home Screen).