It's necessary. Phones are far worse than prior to phones, and I've seen both sides of it.

Boring lessons is a fact of life. People need to realize that not all education can be interesting. Sometimes you gotta suck it up, sit down, and learn. And I say this as someone with crippling adhd

My classrooms banned phones so I played games on my graphing calculator or did the old fashioned drawing.

day-dreaming isn't intrinsically bad. People do need time to think about stuff, and have their mind drift from topic to topic. Some modern teaching practices advocate deliberate "brain breaks" for students.

The issue with phones isn't so much that students are sometimes off task, but rather that the phone consumes their attention entirely. It uses up the students' useful concentration as well as their 'rest' time.