In our local schools, they don't make *teachers* responsible for enforcing the bans. Students have to keep their phones in a Yondr pouch. If they're caught with their phone it will be confiscated (and require a parent to pick it up), and the administration will give also give additional consequences such as being banned from extracurriculars or school activities like Prom.

My student tells me that in practice many students don't keep their phone in the pouch, but they are very careful about how and when they use them. Many teachers have a "don't ask, don't tell" policy - if I don't see you using the phone, and it's not disruptive, then they don't care.