I have a bit more sympathy than that. It is not obvious whether a phone is off or just playing dead.
During a recent league game I was suddenly struck by the thought that I could not remember turning off my own phone, which was in my coat pocket on the back of my chair. I asked my opponent, on my move, whether he would mind if I checked. Of course he didn't, it was on, I turned it off with no harm done. Is there any way I could have handled that at Hastings?
Another question. If I put my phone in a supplied bag and leave it at the arbiters' desk, then it goes off, am I in breach of the AC regulations? I feel that would be unfair, although I should certainly suffer some penalty for disturbing the tournament. To me, if your phone actually goes off in the playing hall, that is an indication that you are either not a cheat or a very poorly organised one.