When I was in high school I always began the rehearsal process by requesting that they keep their cellphones on silent, and not use them when working on stage. This eliminated a lot of my problems right off the bat, and I feel it was better than taking the phones away- gave them more control and responsibility. In the few instances where actors were consistently breaking this request, I would personally pull the actor aside and ask them to refrain from phone usage during rehearsals, letting them know that if they continue to interrupt the rehearsal process then if may become necessary for them to leave their phone with myself/ASM during rehearsals. During performances I always held an "absolutely no phones past the dressing rooms" rule, and did have a box I kept with me for valuables, etc, if it made them more comfortable.
Now in college the actors very much treat the rehearsals as their job (as they well should), and therefore I have yet to need to request them to keep their phones from becoming a distraction in rehearsals. The SM team does reserve the right to find a phone that keeps going off, turn it off, and hold it until the next break is called and the actor can retrieve it. We have the same rule for performance here as I did in high school, and the actors are great about it. Assigned crew... not as much. I've found that as long as you are diplomatic, and keep it general and about the needs of the show rather than a personal attack (if it's phrased as "you aren't doing your job because you're always on your phone, turn it off!", a positive response will likely not be achieved).