How do you prevent people from having phones while inside?

Do you just get in trouble for whipping it out? Or do you have to drop it off with a phone valet at the entrance? If so, how do you prevent theft or mixups? Are all the staff comfortable confronting people who have taken their devices out, risking their tips and personal comfort levels? What if somebody gets cranky after being asked because they didn't know and it's halfway through dinner?

It's a tricky policy to enforce smoothly