Apparently if you are over 20 years old you cannot hear attached sound and it's been used as phone ringtone among youngsters.
I have tested it! I sort of hear some very very very tiny sound and couple of 10 years olds told me that they can hear it fine.
18000 Hz Sine Wave.mp3 (157.52 kb)
What is sine wave? See http://en.wikipedia.org/wiki/Sine_wave