How does a telephone work?

A classic desk phone has very few parts, and none of them is a computer. You hold the handset. Its mouthpiece holds a microphone that turns your voice into a changing electric current, and its earpiece holds a tiny speaker that turns current back into sound.

When the handset rests on the cradle, it presses down two plungers. They work the hook switch. Lift the handset, the plungers pop up, the switch closes, and current starts to flow from the exchange. That current is how the exchange knows you want to make a call.

On the front is the rotary dial. Inside are two metal bells and a clapper worked by an electromagnet, and a small network: a coil and a capacitor that stop your own voice from roaring back into your ear. The whole phone hangs on a line cord: just two copper wires that run all the way to the exchange, which also powers the phone. That is why an old landline still works in a power cut.

Sound is a wave of tiny pushes and pulls in the air (see WaveClear). In the mouthpiece, those pushes hit a thin diaphragm. Behind it sits a little cup of carbon granules, like coarse black sand, between two metal plates.

The exchange's battery drives a steady current through the granules and round the loop. When your voice pushes the diaphragm in, the granules are squeezed together, they touch at more points, and their resistance falls. More current flows. When the air pulls back, the granules loosen and less current flows. So the current in the wire rises and falls in exactly the same pattern as your voice. That copy is called an analogue signal. Thomas Edison's carbon transmitter of 1877 made this work well, and phones used carbon microphones for about a hundred years.

At the far end, the changing current flows through a coil in the earpiece. The coil is an electromagnet (see MagnetismClear and CurrentClear) wound on a permanent magnet. More current, a stronger pull on a steel diaphragm; less current, a weaker pull. The diaphragm shakes the air, and your ear hears the voice again (see EarClear). The permanent magnet matters: without it, the pull would be the same whichever way the signal swung, and the voice would come out garbled.

In between, the exchange feeds each line from its 48-volt battery and passes only the changes across to the other line, through a transformer.

A phone line has only two wires, and they already carry your voice. So how do you tell the exchange which number you want? The first answer was to switch the current off and on.

Put your finger in a hole of a rotary dial and turn it to the metal finger stop. Let go, and a spring turns it back. As it returns, a cam flicks a pair of contacts open and shut, breaking the loop once for each step: 3 breaks for a 3, 9 for a 9 and 10 for a 0. A small governor keeps the speed steady at about 10 pulses a second. The exchange simply counts the breaks. That is why a 0 took the longest to dial!

Touch-tone dialling, first offered in 1963, sends sounds instead. Each key plays two notes at once: a low note for its row and a high note for its column. Press 5 and you send 770 Hz and 1,336 Hz together. The exchange listens for the pair. No pair is a simple multiple of another, so ordinary speech almost never fakes one. It is called DTMF, dual-tone multi-frequency, and it still works today when a helpline asks you to "press 1".

Every phone line in a town runs to a building called the exchange. Its job is to join your pair of wires to the pair of the person you are calling, and then to ring their phone.

The first exchanges were manual switchboards, from 1878. Lifting your handset lit a lamp. An operator plugged a cord into your jack, asked "Number, please?", and plugged the other end into the jack of the line you wanted. Thousands of young women did this job, in Mumbai and Kolkata as in Boston.

In 1891 an undertaker called Almon Strowger patented a switch that needed no operator. In his step-by-step switch, each pulse from your dial kicks an electromagnet. Dial 4 and the wiper climbs 4 levels. Dial 7 and it turns round 7 contacts. It lands on line 47. Bigger towns chained these selectors, one per digit.

Later crossbar switches stored the whole number first and then closed one crosspoint in a grid of bars. Today's exchanges are digital: your voice arrives as numbers (next chapter), and a chip simply copies them into the other line's time slot, 8,000 times a second. Whatever the switch, the called line gets a ringing current, about 75 volts at 25 hertz in India, in a ring-ring… ring-ring rhythm.

Between exchanges run trunk lines. Laying a separate pair of wires for every call between two cities would be absurd, so engineers learned to share one line among many calls: multiplexing.

Modern trunks are digital. Your voice is measured 8,000 times a second, and each measurement is stored as an 8-bit number. That is 64,000 bits a second per call. A multiplexer then lets the calls take turns: one number from call 1, one from call 2, one from call 3, and so on, then round again. This is time-division multiplexing. One round, a frame, lasts just 125 millionths of a second, so every call gets its turn 8,000 times a second and nobody notices the sharing. An E1 trunk carries 30 calls this way, and a single glass fibre today carries millions.

Across oceans, calls first went by radio (1927), then by undersea cable: TAT-1 in 1956 carried just 35 calls across the Atlantic. Satellites came next, but they sit 35,786 km up. Even at the speed of light, the trip up and down takes about a quarter of a second, and people talked over each other. Fibre cables won.

In India, STD (subscriber trunk dialling) began between Lucknow and Kanpur in 1960. In the late 1980s and 1990s, yellow STD/ISD/PCO booths sprang up in every town and village, many run by young or disabled owners, and for the first time millions of people could dial any city, or any country, themselves.

Most "landlines" today are not copper all the way. In many homes the phone plugs into a small box, an ONT, at the end of a fibre-optic cable, and your voice travels as VoIP: voice over internet protocol. The phone still samples your voice 8,000 times a second, but now it packs 20 milliseconds of it into each packet and sends 50 packets a second, mixed in with web pages and videos.

Why does a phone call sound "telephone-y"? The classic channel only carries sounds from 300 to 3,400 Hz. That was enough to understand words, and it kept each call cheap: 8,000 samples a second can only describe sounds below 4,000 Hz. It cuts off the deep part of a voice and the hiss of s and f. Oddly, you still hear a man's low pitch, even though it is below 300 Hz: your brain works it out from the spacing of the harmonics. Newer HD voice calls carry 50 to 7,000 Hz and sound far more natural. The same narrow channel also carried fax machines, which turned pages into tones (see FaxClear), and the dial-up modems of the early internet.

There are trade-offs. An old copper phone is powered by the exchange's batteries, so it works in a power cut. A fibre box needs power at home, or a battery backup. Landlines are fading worldwide, from 20 per 100 people in 2005 to 11 in 2023, as mobiles take over (see MobileClear). Whatever phone you have, remember 112: it reaches police, fire and ambulance anywhere in India, and across Europe too.