Telephones connect people around the world, yet they're one of the simplest devices in a home. While telecommunications systems have changed dramatically, the simplest telephone contains the same three components used in the 1920s.

Telephones require a hook switch to connect and disconnect the phone from the network, a speaker and a microphone to carry sound waves from your voice. Phone numbers on these simple phones can be dialed using the switch. To dial the number four, for instance, you would simply tap the switch hook quickly four times. The whole setup is so simple that you can build your own phone line at home using wires, a battery and a resistor.

Most phones are slightly more complicated.  Besides the basic components, they contain duplex coils to block the sound of your own voice from reaching your ear, a bell that alerts you about an incoming call, a touch-tone keypad and a frequency generator.

An electronic switch that produces dial tones and busy signals has replaced the telephone operator, and the microphone also is electronic.

Telephone networks begin in your home. A pair of copper wires runs from a box at the road to a box at your house. The wires are connected to each phone jack in your house. A cable containing more than 100 pairs of copper wires runs along the road. The cable runs to the phone company's switch or to a nearby box, called a concentrator digitizer that digitizes your voice and combines it with others. Together, the voices are sent down a single wire to the phone company. Your voice usually travels over a fiber-optic line to the person you're talking to, but it may also be transmitted by satellite or by microwave towers.