When it came out in Britain in December, the Mosquito sound system was supposed to be the sonic equivalent of a "no loitering" sign. Its annoying, high-pitched sound -- which many adults can't hear but most young people can -- would act as a teen repellent.

Now, teens are staging a worldwide rebellion: Downloading the sound, or another ring tone in that same high-frequency range, allows them to hear their cellphones ring when their parents and teachers (mostly) cannot.