My experience is USCF says to turn phones off. FIDE doesn't allow them in the playing hall at all.
I've never suspected an opponent of cheating, but sometimes I think about my own bathroom timing. For example I'd rather not use the bathroom in the middle of unclear tactics just so I don't make my opponent suspicious. Maybe I worry about the appearance too much, but anyway, thats the extent of it for me.
It's a local club with small prizes though, so I'm not worried about it. Maybe if I were playing in one of the huge tournaments where even low rated player might win thousands of dollars I'd start to worry.
The last time I played there was no Stockfish.
Are phones banned from the playing hall?
Does cheating happen regularly?
Have you suspected your opponents of using an engine while on a bathroom break?
How big a problem is it?