Again: the problem was enforcement. It turns out that making 1000+ teenagers put their phones into bags (and ensuring that they don’t open them) is not trivial.
The problem with contact is when the student leaves the school: the phone was typically confiscated for multiple days, meaning that students would be left without their phones when they left the grounds. Many parents give their children phones so they can reach them if they’re lost or similar.
Notably, the city tore up all of the payphones around the same time.
Edit: as a piece of trivia: prior to the end of the ban, there was an entire thriving industry of phone escrow vans parked outside of schools[1].
Why not confiscate the phones for only the length of the school day and give them back when the student leaves? If you need a harsher punishment as escalation, use something unrelated to the phone like a detention or suspension.
At my son's school they have to put their phones in the 'phone hotel' at the beginning of the school day and pick them up at the end. The kids just bring two phones and put a junk one in the hotel. If they were to enforce it with detention or suspension, 75% of the student body would permanently be on detention/suspension.
School has breaks too. What are you doing if a teacher is absent or you have something like a 3h lunch break?
Growing up you were simply not allowed to actively use your phone during class, outrightly banning phones from school makes no sense. We'd also use them for some class activity at least once a week.
Banning cellphones in schools is very practical. You put them into the pouches they use at concerts. Pouches are opened at the end of the day.
If parents need to reach their kids they can do so easily. Call the school and ask for the kid.