Our mobile phone policy is written into the contracts. No use of phone on any job unless it's an emergency or unless someone from the company is calling. If you let one ring or text for 10 mins on a job and another worker gets wind of it, they will want a 10 minute break. Then you are on a hiding to nothing because no one knows the rules. Get it written down, then everyone knows. You can discipline if someone breaks the rules too.
You tend to find that people with 9 points on their licence drive very carefully...