I used to work for a K-12 in IT support. School districts have systems or subscribe to services to blast-out notifications. Back when I was in it, those were voice messaging systems that would read out a prerecorded message to the phone of the guardian(s) on record. More modern systems may include SMS capability or even an app that ostensibly is for parents to interact with teachers, but supports more notification capabilities than just that.
Around fifteen years ago a new deputy superintendent tried to push for a BYOD policy for student devices, up to and including phones and obsolete PDAs like older Palm devices. The must've been pretty slick showing those old devices doing something meaningful because there was a hard push to make this happen. Ultimately where it was piloted it was basically not used, either because the kids didn't have personal devices to bring to begin with, or because the kids were doing anything except their actual lessons.
I would agree that there's basically no benefit to having cell phones during instruction time. For old kids, save 'em for passing periods, lunchtime, and before or after school. For younger kids, just don't even bother having them during school hours.