# How Telephones Work

Tech | Telephone Technology

## Telephones: Tones

In a modern phone system, the operator has been replaced by an electronic switch. When you pick up the phone, the switch senses the completion of your loop and it plays a dial tone sound so you know that the switch and your phone are working. (For more information on tones, see How Guitars Work.) The dial tone sound is simply a combination of 350-hertz tone and a 440-hertz tone, and it sounds like this.

You then dial the number using a touch-tone keypad. The different dialing sounds are made of pairs of tones:

1 = 697 Hz + 1,209 Hz

2 = 697 Hz + 1,336 Hz

3 = 697 Hz + 1,477 Hz

4 = 770 Hz + 1,209 Hz

5 = 770 Hz + 1,336 Hz

6 = 770 Hz + 1,477 Hz

7 = 852 Hz + 1,209 Hz

8 = 852 Hz + 1,336 Hz

9 = 852 Hz + 1,477 Hz

* = 941 Hz + 1,209 Hz

0 = 941 Hz + 1,336 Hz

# = 941 Hz + 1,477 Hz

A typical number that you dial sounds like this:

If the number is busy, you hear a busy signal that is made up of a 480-hertz and a 620-hertz tone, with a cycle of one-half second on and one-half second off, like this: