and Test Tones
|1209 Hz||1336 Hz||1477 Hz||1633 Hz|
The frequencies were chosen to avoid harmonics : no frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies. Although DTMF works in normal human voice range, the combination of inharmonic frequencies makes it hard for the human voice to impersonate.
Although mobile phone networks use digital signals instead of DTMF for direct dialing, DTMF is still used over mobile phones to navigate automated systems such as phone menus, voice menus and other advanced calling services.