When you dial a number from your telephone, you can hear a different sound for each button you press. These tones are called DTMF (Dual Tone Multiple Frequency) codes as each sound consists of two tones modulated together.

Here's a list of the frequencies used. The official minimum on/off times are 100msec pulse widths, but most equipment will work with other timings.

Tones are also used for letting the user know if the number they are calling is engaged etc. Here's the list. All on/off times are in msec.