just a bit of background: when humans are youung, we can pick up on higher pitched noises, but as the ear ages, we lose that ability. some people have been testing this device in the UK called the 'Mosquito", which uses these high pitched noises to trive away teenagers (younger folks).
yes it works, but now some crafty people have taken the technology to a new level, and turned it into a ringtone for a cell phone, and if used to alert when you get a text message, students can hear it in class, but the teacher cant.
read about it here
I thought it was interesting how something ment to drive kids away is now being used for them to comunicate in almost a whole different language.
enjoy. -digital 
Every phone ships with Vibrate mode. Also, I tested this theory on my Motorola E815 which has a pretty good speaker.
I am 27 and I can hear 24kHz just fine. Television flyback coil noise, camera flashes charging, and other high-pitched whines drive me nuts. I had to bump up the sampling frequency of my audio-editing software to 96kHz just to get it to generate past 20kHZ without distortion, and even now, I'm not really sure if 24kHz is my actual hearing limit or if the frequency response of my high-end headphones is attenuating the signal to the point where it doesn't actually make any sound. I may wire up some cheap piezoelectric speakers since their frequency range is typically higher.
That said, the speaker on my motorola E815 can't audibly play much higher than 14kHz. I think it would be more likely that this is a hypothetical "I'd bet kids would do this" mindstorm that eventually turned into an urban legend of sorts, because honestly, Vibrate mode is quite a bit more feasible. Just to be sure, I used my cats to test my motorola E815's ringer though. They freak out when I play the high pitch over my headphones, they didn't notice anything when I played it on my phone or on my cheapo computer speakers.
Also, the "mosquito" frequency can't be recorded by means available to most kids, such as cellphones, digital voice recorders, or whatnot. Not only is the sampling frequency (the audio resolution level) of stored audio way too low to accurately represent the high frequency, but the frequency response of the microphones themselves probably is left wanting in this sort of situation. It would more feasibly have to be generated with Audacity or CoolEdit.
Sorry, I declare this myth: BUSTED
Ax0n nails it. Most cell phone speakers cap at around 10 KHz - more than good enough for voice and ringtones.
I did find a link that allegedly contains the ringtone, but could only hear background noise. Yet, I know I can - and just confirmed with a tone generator - that I can still hear 20 KHz, and surprisingly 24 kHz, but only barely. Maybe only some modulation in the tone, as it wasn't a pure tone.
Anyhoo, enough about my own lobes. I suspect this ringtone is based on a frequency closer to 10 KHz, is quiet, and has enough modulation in the sound that it makes a quietly audible ring...but isn't as advertised.
opps, I double posted...
Well, after years of loud music, and fire crackers with short fuses over deep storm drains, and other abuses, I have tinnitus. It is not so bad that I have to have the white noise stuff and go nuts (more than I already am) over the ringing (mostly 'cuz it drowns out the voices... opps, that is another topic for later).
I have a distinct hearing loss around 10k and I hear ringing when in a quiet room or other peaceful place with low noise levels. Should such a device ever come to pass, I am sure to either not notice or become some psychotic paranoid always asking who has the phone call since I would always hear a faint ringing.
So that's what that noise in my head is. Great. I'd been meaning to ask the Dr. about it, but it seems that all the questions I have prepared ahead of time fall out the top of my head when he tells me to bend over (it sucks being over 40!)
I just ran some tests with a sound generator (in Amadeus II), and found that the speakers on my laptop aren't able to reproduce anything above 12kHZ; even my 12 & 16 year old sons couldn't hear anything in that range. With some decent Sony earbuds, I can pick up everything up to about 14kHZ; nothing at all between 15-19, but I can hear 20kHZ to 22kHZ. I think that's about the limit of the earbuds' range.
Back to the original topic, I would agree that AxOn hit the nail on the head, and the story about the mosquito sound being used as a stealth ringtone is total BS. Cute, but it just don't hold water.