Please time-out the ringing on mobile clients

My friends started a group call at 9PM, and I was unavailable. It apparently continued ringing on my iPad, all the way through to the next day, completely draining the battery when I needed it. In my opinion there should be a ringing timeout - If I don't pick up/join the call, the incoming call screen shouldn't be active - Especially if I've got the cover closed! An alternative could be an indicator on the group in the UI that there are active calls taking place.