Cell phones are an interesting debate. In dealing with my professional actors, there isn't a rule forbidding them to not have their cell phones. I do enforce a rule of a silent cell phone (no ringing, no vibrating). However, them having cell phones back stage I can't control not always being backstage. I have never had a noise problem or a distraction problem. They know that during rehearsal and shows when they are on stage the cell phone is away.
I have also been in a position where I was ASMing a show and I got a text message during the show from my production manager. He and the house manager were locked out of the theatre and needed to be let back in. It was a good thing I had my phone on me. Silent of course, and I just happened to see it light up.
Different scenario, I was on headset back stage and had my SM calling cues via headset, but I also was in full communication with my production stage manager via text message.
I also worked another theatre where I had to text message my stage manager about an injury and a set problem because there wasn't a headset available to communicate by. Only sometimes did that work. If it didn't, I would text the spot operator to tell the SM or if I was free to go to the booth I would do that instead.
I also work in different theatres at the same time and I keep everything on my smartphone. I will turn it off if it messes with the sound, but small theatres in the city tend not to use mics so much, so I have only had to do it twice.
As for actors, some keep them, some turn them in as a valuable. I leave it up to them as long as they are responsible about the cell phone handling. If it doesn't effect their work or make noise, they are fine. If it does, I give a warning which usually fixes the problem.