I think you've sort of missed the forest for the trees, here. If you've gotten to the point where the kid is hiding a phone from you, you're already in an adversarial relationship with a young adult who has lost trust in your ability to be a reasonable authority figure.
Pretty much all teens have stuff they are hiding from their parents. Some of it is trivial, some might be pretty serious. For example I'd guess it's not very common for your kid to come home and announce he's started having sex (though one of mine did).
Ha! my dad celebrated my announcement by inviting me to a good restaurant. I would never have thought about hiding that from my parents and I had their full support to bring my girlfriend back to my room (and later for the night)
You seem to think that there aren't any kids that, under pretty much any circumstances, would want to have a private device. Speaking from experience (having been a kid myself) I can assure you that these kids exist.
