There are 3 reasonable things user may want to do in context of an incoming call: accept, reject and, possibly, move the widget.
If the whole surface of the widget is "draggable", there is no need for additional "chrome".
Assuming that, in most cases, user will want to accept the call, the surface of the widget can be made into a large "Accept" button. If user can't talk right now, there's a smaller "Decline".
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