for mobile phones predominantly... modern smart phones...
Phones have accelerometers these days. They can detect when they're moving.
So what do you do when someone calls? I think most common is:
- pick up phone - lifting it upwards
- check the caller on screen, holding phone stable
- and then either:
- lift phone again to your ear to talk
- put phone back down to hang up.
- Lift-pause-lift = pickup
- lift-pause-lower = cancel.
Maybe the cancel should show an on-screen countdown (3 seconds?) before actually cancelling the call, since odd locations (laying down) could cause the final lower to be for picking up... of course, there should always be buttons which can be activated to force hangup/cancel regardless.
So you then have... pickup/cancelling calls without the need for ANY buttons to be used at all! :D