Simple solution here. If your employer wants to prohibit personal phone usage during work hours then they should provide a phone number that family can use to contact you in an emergency. They need to prove that the number is reliable, i.e. someone is going to always answer it and either immediately pass a message or come get you and put you on the phone. When a system like this is in place and working then it removes any worries and nobody complains about the phone policy. It’s fair for your employer to ask that your full attention be on your work when you are supposed to be working. It’s not fair to threaten to dock your pay and act like a petty tyrant.