Apparently children are using the noise that was designed to be audible to them, but not to adults, as ring tones. The noise was created to discourage children from congregating in places like shopping centres. However children have now started using it as a ring tone on their mobile phones. That way teachers (or at least most teachers) will be unable to hear it, whilst they can.
Besides showing the ingeniousness of children it also is a wonderful demonstration of the law of unintended consequences.