.

I use the same motto on my kids that my mother used for me: if you can't do it in front of me then it must be wrong. In other words, if you have to hide and don't want me to find out, then maybe you shouldn't be doing it.
This is what I am referring to. Your ds is still abiding to the rule because he's not using it per-say. If he's going to lie to you, there are bigger problems. You should be able to trust them.

If they aren't using it for an alarm though, why do they need the phone?