My daughter has to leave her cell phone at home, but she's in junior high.
Just send out a reminder notice that hourly personnel are welcome to use their cell phones during break and lunch periods, but not at any other time. Then, zero tolerance.
The worst thing you can do is make a rule, then not enforce it because when you do begin enforcing it they will say: "You never said anything to Bob, Carol, Ted, or Alice and they did it
all the time and I just did it this once!"
Thanks for the reminder... for years, the only personnel problem I have had is the cat walking across the keyboard and I have forgotten what a plus that is.