In a modern phone system, the operator has been replaced by an electronic switch.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. The dial tone sound is simply a combination of 350-hertz tone and a 440-hertz tone, and it sounds like this:

• Click here to hear a dial tone.

(For more information on tones, see How Guitars Work.)

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

A typical number that you dial sounds like this:

• Click here to hear a touch-tone number.

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:

• Click here to hear a busy signal.