The public switched telephone network, commonly called the PSTN, is the global system of copper wires, fiber-optic cables, switching centers, and signaling protocols that has carried voice calls for over a century. When you pick up a traditional landline phone and dial a number, the PSTN is the infrastructure that connects your voice to the person on the other end by creating a dedicated circuit between the two phones for the duration of the call. It remains the backbone of voice communication in much of the world, though most countries are now actively replacing it with internet-based alternatives.
How a Call Travels Through the Network
Every traditional phone call starts at the local loop: a pair of copper wires running from your home or business to the nearest central office, sometimes called an end office. These wires carry a small DC voltage, typically between 38 and 78 volts when idle, which drops when you pick up the handset. That voltage change signals the central office that you want to make a call. The current flowing through the line during a conversation stays between 18 and 55 milliamps, which is enough to power a basic phone without any external electricity. This is why old-fashioned landlines still work during a power outage.
Once you dial a number, the central office determines the best path to reach the destination. If you’re calling someone served by the same office, the connection is simple. If the call needs to travel farther, it gets handed up through a hierarchy of increasingly powerful switches until it finds a route to the other person’s local office, then back down to their phone.
The Switching Hierarchy
The PSTN organizes its switches into five classes, numbered 1 through 5. Your call enters at a Class 5 switch, the local central office closest to your phone. This switch processes incoming calls, figures out where they need to go, and establishes a circuit connection. For calls within your local area, this office handles everything.
Long-distance calls move up the ladder. A Class 4 toll center takes over when the call needs to travel between regions, connecting to the long-distance carrier’s network. If that toll center can’t find a direct link to the destination, the call escalates to a Class 3 primary center, then to a Class 2 sectional center, and finally to a Class 1 regional center at the top of the hierarchy. Each level covers a broader geographic area, and calls always travel as few steps up the chain as necessary before being routed back down to the destination’s local office.
In practice, modern fiber-optic trunk lines between switching centers mean most calls don’t need to climb all five levels. The hierarchy exists as a fallback, ensuring there’s always a path available even when direct routes are congested.
How Switches Talk to Each Other
The switches across the PSTN coordinate using a protocol called Signaling System 7 (SS7), which handles call setup, routing, and billing. SS7 operates on a separate channel from the voice call itself, which is why it’s called “out-of-band” signaling. Think of it as a private messaging system that switches use to negotiate connections before your voice ever travels the line.
When you dial a number, your local switch creates an initial address message containing the calling number, the destination number, and which trunk line (the connection between switches) it plans to use. That message travels through the SS7 network to the destination switch. Once the far-end switch receives it and locates the person you’re calling, it sends back an address complete message confirming the route is set. When the other person picks up, their switch sends an answer message, and billing begins. When either party hangs up, a release message frees the trunk line for other calls.
SS7 also handles more complex tasks like translating toll-free numbers. When you dial an 800 number, your switch essentially asks the SS7 network, “Where do I actually route this?” and gets back the real destination.
The International Numbering System
Every phone number on the PSTN follows a structure defined by an international standard called E.164. The maximum length for any international phone number is 15 digits, not counting the international dialing prefix (like 011 in the US or 00 in Europe). Each number starts with a country code, which ranges from one to three digits. The remaining digits form the national number, which identifies the specific subscriber within that country.
This standardized system is what makes it possible to call virtually any phone on the planet from any other phone. The country code tells the network which nation to route toward, and the national number narrows it down to a specific local switch and subscriber line.
What Makes the PSTN Distinct
The defining feature of the PSTN is circuit switching. When you make a call, the network dedicates a continuous electrical path between you and the other person for the entire conversation. No other call can use that circuit until you hang up. This is fundamentally different from the internet, which breaks data into packets and sends them along whatever route happens to be available at that moment.
Circuit switching is less efficient with bandwidth, since the circuit stays open even during silences in conversation. But it delivers consistent, predictable voice quality. There’s no buffering, no dropped audio from network congestion, and virtually no delay. The PSTN’s two strongest characteristics are reliability and stability, qualities that internet-based calling still struggles to match consistently. Voice-over-internet services can suffer from frequent disconnections during calls, particularly when network conditions are poor.
The other major advantage is power independence. Because the copper local loop carries its own voltage from the central office, a basic landline phone works without household electricity. Central offices maintain large battery banks and backup generators, so the phone system can keep running through extended power outages. Internet-based phones, by contrast, go down when your router loses power.
The Global Shutdown of the PSTN
Despite its reliability, the PSTN is being retired worldwide. Maintaining aging copper infrastructure is expensive, and modern fiber and wireless networks can carry voice as data far more efficiently. The transition is happening on different timelines depending on the country.
In the United States, the FCC issued a mandate requiring service providers to replace traditional copper phone lines with digital alternatives, with a target of completing the transition by the end of 2022. In practice, the shift is ongoing, with carriers gradually migrating customers to fiber or wireless connections. The UK has set a firmer deadline: legacy PSTN technology and its associated infrastructure will be made permanently inoperable by December 2025. Australia and New Zealand are phasing out landline service 18 months after broadband becomes available in each area, though no single nationwide cutoff date has been announced.
For most people, this transition is invisible. Your phone number stays the same, and calls still work. The difference is behind the scenes: your voice gets converted to digital data packets at some point in the journey rather than traveling as an analog signal over a dedicated copper circuit from end to end. Many “landline” phones today already connect through a digital box provided by a cable or fiber company rather than plugging directly into a copper wall jack. If that describes your setup, your calls are already traveling over the internet for at least part of their route, even if it doesn’t feel any different from dialing a phone in 1985.