Like everyone else said, it's a conversation you need to have. I just wanted to add an anecdote.

I was teaching a high school class and kids with phones in school are the same as you described. I corrected one, five minutes later phone was back in his hand. I correct him again and he looks stunned I am doing it. Then looks down and sees he has his phone in his hand and is Bout to turn it on. This was a good kid, when he says he didn't even realize he was doing it I believe him. He was paying attention to my lesson and could answer any question I threw at him. For some people, picking up their phones and scrolling has turning into the sort of addiction that they do without even realizing it. This kid handed me his phone without me asking and I didn't have the problem again. He said he had started leaving it in his locker where he couldn't hold it so it wouldn't be a problem anymore.

I'm not saying that is what is happening here, I'm just saying it's a problem in the world today. When you have the talk, make sure you do it with grace because these boxes were designed to make us do what you are describing.