Cell phones caused so many problems in classrooms when I was an Education Assistant. The students would hide them under a book, or have them on their knees under the desk. The rule back then was simple, they were not be used in the classroom, period!
If a student was caught using their cell phone, staff would take it, out the kids on it, and it went to the office to be picked up by the kid after school. Repeated offences were given to the admin to deal with; many students would argue that we did not have the right to take their phones, many parents would storm into the office and berate the secretary and whoever else was there at the time.
The year I retired the rules changed and a teacher could allow the phone as a learning tool. I remember saying how glad I was that I was retiring because this new rule would never work.
Kids are devious, they will always find a way to use their phone. Girls would ask to go the wash room, they would take their bags with them, guess what was in their purses? Cell phones! I have caught many a young lady who apparently was desperate to use the wash room just having a nice little chat with a friend, who was in another class or another school.
The only thing to do is ban them from the classroom altogether. If there is an emergency and a parent needs to contact their student, they can call the office and the secretaries will get a message to the kid. Cell phones ring in class, they are disruptive, especially in the middle of a test or when the teacher is teaching. Make one rule and stick to it, make it clear there will be consequences and follow through.