Phone use during work

For context: I manage a bar/restaurant. And there's always something to do in hospitality. If there's no customers, there is plenty of prep to make or glasses to polish.

Today was a dead day for us. No customers at all for 90% of the time. I caught my waitress on her phone while doing tasks I've assigned her. At some point it was 50% using her phone and 50% doing the task I told her to. I pointed it out and told her that in some other venues the manager takes the phones away. She became defensive and said that workers would leave such a venue and that it's worker's abuse to take their property. She said that she doesn't use her phone when we have customers in. She still continued to use her phone all the time. I was moving around the venue and every time I wasn't near her, I could see she was on it.

At some point I brought out a contract that everyone signs and showed her what our policy says about phone usage. Which is to keep it in the locker room. She told me I'm being annoying. My point is: when you're on your phone you're less productive. I have to be on my phone all the time as a manager, because i need to talk to the owner and to the other employees to adjust the roster. Doesn't mean everyone else can chat away on their phone.

Am I being the asshole here? What's the phone policy at your venue/workplace for the employees?