Young people have neatly reversed the purpose of a technology which has been used - controversially - to deter them from congregating in public spaces like shopping malls. According to Wikipedia, the UK leads the world in the use of this technology although it is opposed by the Children's Commissioner for England, Liberty and the National Youth Agency. The ring tones have been known about for some time but are now widespread.
You can hear samples of the tones (or not!) on the Mosquito Ringtones website where they are advertised as "ring tones teachers and parents can’t hear!". They are free for download.