Should a parent be allowed to look through their teenager's phone?

Personally, I think it's a total invasion of privacy that only teaches the kid, quite accurately, that they can't trust their parent, and that their parent doesn't trust them. I know from experience how damaging it is for a parent child relationship to lose it's trust.

I'm interested to hear other perspectives though, especially from parents.

EDIT 1: On the note of who's paying, it doesn't matter. Once you give the phone to the kid, it's either theirs, at which point they own it and it doesn't matter whether the money came from the parent, then child, or a damn alien, [i]they own the phone[/i]; or the phone is effectively on loan from the parent and so isn't owned by the teenager, which is just a [i]great [/i]way to teach your kid that you trust them. [i]Totally great[/i].

Talking about cost just dodges the moral question, and replaces it with one about ownership that is totally irrelevant.