In order to allow more long-distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3,000 hertz. All of the frequencies in your voice below 400 hertz and above 3,400 hertz are eliminated. That's why someone's voice on a phone has a distinctive sound. Compare these two voices:
You can prove that this sort of filtering actually happens by using the following sound files:
Call up someone you know and play the 1,000-hertz sound file on your computer. The person will be able to hear the tone clearly. The person will also be able to hear the 2,000- and 3,000-hertz tones. However, the person will have trouble hearing the 4,000-hertz tone, and will not hear the 5,000- or 6,000-hertz tones at all! That's because the phone company clips them off completely.
For lots more information on telephones, telephone networks and related technologies, check out the links on the next page.