Belgium here, high school teacher... Our school banned smartphones two years ago. 12 to 15 year olds have to hand them in. The older ones have to put them somewhere where it is out of hand's reach. Parents rejoiced, kids were disappointed. Now it is routine. Colleagues teaching the young ones say the focus is a lot better. In the higher grades, where I teach, I notice little difference. I have to add though that students in our school are generally... more well behaved than in average schools.
Government mandated a phone ban last year. A (reliable) newspaper reported that screentime remained the same. Meaning that kids use their phones more at home now.
Personally I have mixed feelings about this. I am pro the ban. Especially for young kids. I see smartphone zombies at every bus stop that I pass by car every morning. It is a sad view. Then again, us adults have little credibility there. We are not giving a good example. My son was at a music camp. I went to get him. Most parents were clinging to their smartphones while waiting for the kids to come out. If we make rules for the kids, we should also make rules for parents.
In short, smartphone ban has some advantages. It is not a miracle solution though. It is easy to bash kids. Lead by example...