Coordinate everything. If your teen has no cell phone, they have no access to Church, school, etc. At least at my kids school, starting in 7th grade, everything shifts to Cell Phones.
My daughter used her phone and Google Drive for Yearbook. She reads books for class on overdrive. She turns in assignments via her phone. Checks her grades, checks her assignments.
The world when you were young was not designed around cell phones. The world today is.

I can't imagine a teen being able to do much of anything without one these days.