My oldest son mentioned this to me the other day: a 'silent' ring tone for a cell phone that adults cannot hear.
Naturally, I thought he was pulling my leg; along the lines of 'the dog ate my homework'. Only if you coat it in cheese, son.
However, he wasn't. There is a 'mosquito' ring tone out there, per MSNBC: “Students find ring tone adults can't hear”. The article substantiates his claim.
Haven't these kids heard of vibrate mode?
Update: I found a URL on NPR for the ringtone and posted it in: The 'silent' ring tone.