Put your phone in your pocket at dinner and watch everyone else quietly follow. Norms start with one person.

Group behaviour is mostly everyone copying whoever moved first. Nobody at the table especially wants the phones out; they are matching the room, and one different signal is enough to reset what people assume is normal. It works without a word being said, and it stops working the moment it turns into a lecture.

Try it once, announcing nothing and saying nothing when others follow. Notice how much longer the conversation runs. If someone has to check something, let that be fine, since the aim is a better hour rather than a rule.