I use my cellphone at work but solely for work-related things like finding discount cards, texting my rxm or reading the 15 emails a day we get from our our boss. If I'm completely caught up, I mighttt text a family member once or twice during my 12 hour shift.
Basically, I set the example, so if we're slow I don't mind having their phone out as long as NO customers are around (almost never) or they go hide out of sight if they need to take any important calls during a long shift(kids, family issues). Even as a staff, you're ultimately the manager during your shift so just let them know your expectations for professionalism.
I had a convo my first week and asked them what their previous expectations were with cellphone use when I noticed they were using them often. They said they weren't allowed at all with previous PIC. I told them my expectations and they quickly complied. I do have a weird relationship with my techs where we treat each other like garbage on purpose but we work really well together, so the usual "quick slacking off and get your a-- back to work" does the trick if it presents itself.