How Does a Phone Call Actually Work?

VoIP From the Ground Up: Part 1 of 10

Before you can understand why internet phone calls sometimes sound terrible, you need to understand how phone calls worked before the internet existed. The original system was elegant, simple, and incredibly reliable. It also explains why we eventually had to move past it.

A wire between two people

At its most basic, a phone call is just two people connected by a wire.

When you speak into a telephone handset, the microphone inside converts the vibrations of your voice into an electrical signal. That signal travels along a copper wire to the other person's handset, where a small speaker converts it back into sound. The whole thing happens in real time. There is no recording, no file, no waiting. The electricity moves and the other person hears you.

This is exactly how Alexander Graham Bell's original telephone worked in 1876, and the core idea stayed the same for over a hundred years.

The problem of scale

A single wire between two people works fine. But what happens when you want a thousand people in a city to be able to call each other?

You cannot run a dedicated wire from every phone to every other phone. For just 1,000 phones, that would require nearly 500,000 separate cables. So instead, the telephone industry built central offices. Every phone in a neighborhood connected to a local central office with one pair of copper wires. When you picked up the phone and dialed, the equipment at the central office created a temporary connection between your wire and the other person's wire.

In the early days, human operators physically plugged cables together on a switchboard to make this happen. Later, mechanical switches automated the process. Eventually, electronic switches took over. But the concept never changed: pick up the phone, the network builds a dedicated path to the other phone, and that path stays open until you hang up.

Circuit switching

This approach has a name: circuit switching. When you make a call, the network creates a circuit, a continuous electrical path from your phone through one or more switches to the other phone. That circuit is reserved exclusively for your conversation for the entire duration of the call. Nobody else can use it. It does not matter if you are talking, listening, or sitting in silence. The circuit stays open and dedicated to you.

This has some real advantages. The connection is consistent. There is no competition with other traffic. The audio quality depends almost entirely on the physical condition of the wire and equipment. If the call sounds good when it starts, it will sound good the whole way through.

But it also has a major drawback: waste. Studies have shown that during a typical phone conversation, actual speech is happening less than half the time. The rest is silence, pauses, listening. With circuit switching, all of that silence is consuming the same resources as active speech. The circuit does not care whether anyone is talking. It is locked in place either way.

The network that held it all together

The traditional telephone network, known formally as the Public Switched Telephone Network, grew into one of the most reliable systems ever built. At its peak it connected billions of devices across the planet with an expected uptime that most modern technology still cannot match. When you picked up a landline phone, you got a dial tone. Period. Power outages did not matter because the phone line carried its own power. Internet problems did not matter because the internet was not involved.

This reliability was not an accident. It was the result of a century of engineering focused on one thing: making sure voice calls just worked. The network was purpose-built for voice. Every piece of equipment, every protocol, every design decision was optimized for carrying a live human conversation from one point to another.

So why change?

If the traditional phone network was so reliable, why did we move to something else?

Cost and flexibility.

Circuit switching is expensive. Maintaining a global network of copper wires, central offices, and switching equipment requires enormous ongoing investment. Long distance calls were historically expensive precisely because they required dedicated circuits across long physical distances.

Meanwhile, by the 1990s, another type of network was growing fast: the internet. And the internet used a fundamentally different approach to moving information. Instead of dedicating a circuit to each conversation, it broke data into small pieces and sent those pieces independently across whatever path happened to be available. This was vastly more efficient. The same infrastructure that carried a web page could also carry an email, a file transfer, and, eventually, a voice call.

The promise was compelling. If voice could ride on the same network as everything else, companies could stop paying for two separate systems. One network for data, one for voice. Why not just use one?

That question launched an entire industry. It also introduced a set of problems that the traditional phone network had already solved, problems that VoIP users are still dealing with today.

What comes next

In the next post, we will talk about what a network actually is and how data moves across one. Understanding this is the foundation for understanding why voice on the internet behaves differently than voice on a dedicated phone line.

This is Part 1 of the VoIP From the Ground Up series. Part 2: What Is a Network, and Why Should You Care? covers how modern networks work and why they handle data differently than the old telephone system.