Compliments are free and easy. Use them when earned, whether this is a good catch on a negative blocking rig, a smooth day overall, or whatever. Just make sure it is proportional to the achievement.
This balances the criticism. Make sure the implications of the action being criticized are concrete and spelled out, so its not just you "b*&ching" for not real reason.
If he is your employee, then plain and simple, "You have shown that you can't balance using your phone at an appropriate time in work. You can use it during breaks and lunch only. The next time you are doing something dangerous like not paying attention because of the phone during work, you will be fired." Document these past problems in writing. Have a place for his written response on your Disciplinary Action Form (start with something simple if you don't have one). Document what will happen differently (no use during work), and the repercussions if he doesn't stop (termination due to unsafe work practices and not working during work time). Make sure that there are appropriate times during the day for him to use the phone if they will be allowed on the worksite (15 min break in morning or afternoon, and a scheduled lunch).
I let my guys use their phones within reason, but not during the middle of rigging operations, etc.
They can always say to me, "Is this a good time to make a 5 minute call?" or "I need to text someone something. Is this a good time?"
Its easy to say 'yes', or 'no', or 'let's get this top out of the tree before the wind picks up, then we'll stop for a few minutes (or 10 or 15) break.'
If he's not your employee, you can just tell the boss that you are concerned about productivity, as you can't work while your ground support is not ready to work safely, as its a team effort. You are concerned about both safety and productivity, and you need to let the boss know that there is a problem. You don't see the need for the boss to pay you both, plus insurance... for him to be texting people.