I've received quite a few emails about this. A British paper, metro.co.uk, reports that resourceful teenagers have devised a way to make the ringing of their cellphones inaudible to adults. The trick is that they've recorded the sound of the Mosquito, which is a device that emits ultrasonic tones inaudible to most people over the age of twenty-five, but quite audible, and rather annoying, to people under the age of 25:

Techno-savvy pupils have adapted the Mosquito alarm, used to drive teenage gangs away from shopping centres. They can receive calls and texts during lessons without teachers having the faintest idea what is going on. The alarm, which has been praised by police, is highly effective because its ultra-high sound can be heard only by youths but not by most people over 20. Schoolchildren have recorded the sound, which they named Teen Buzz, and spread it from phone to phone via text messages and Bluetooth technology. Now they can receive calls and texts during lessons without teachers having the faintest idea what is going on.

Can this be real? Well, the Mosquito is real enough (I posted about it back in November 2005), and it is true that young people can usually hear higher frequencies than older people because we lose the ability to hear high frequencies as we age. The question is whether cellphone speakers can generate these ultrasonic frequencies. Boing Boing posts a comment from one of their readers who swears that cellphone speakers would not be able to do this. However, another reader links to an article that contains an mp3 recording of the Mosquito sound. (When I listen to it I can't hear any high-pitched noise, just a bunch of street noise.) So if computer speakers can generate these frequencies, perhaps some high-end cellphone speakers also can. Seems plausible. In other words, I'm not yet willing to label this story as a hoax or false rumor, even though it does seem to be a bit far-fetched.