What is the PSTN (Public Switched Telephone Network)?

Public means the network is open to anyone who subscribes and connects them to subscribers on other carriers. A company's internal phone system is private by comparison, even though it links to the public network.

Switched means the network makes connections on demand. You don't have a fixed wire to each person you might call. You dial a number, and the network finds a route to it for the length of the call. Telephone means it was built for voice, though fax machines, alarm panels and dial-up modems were later designed to use it too.

US rules define it along the same lines. 47 CFR 20.3 describes the public switched network as any common carrier switched network, by wire or radio, that uses the North American Numbering Plan. It names local exchange carriers, long-distance carriers and mobile service providers. That is a US rule, and other countries write their own. It does show why treating the PSTN as another word for landlines gives the wrong picture, because mobile networks sit inside the definition.

A shared numbering system is what lets many separate networks behave as one. Carriers draw their numbers from a shared plan, and the plan gives every network a common way to address a call.

The United States shares its plan with 19 other countries. NANPA, the plan's administrator, describes the North American Numbering Plan as serving 20 countries under country code 1. Its numbers have ten digits: a three-digit area code followed by a seven-digit local number. NANPA assigns the numbering resources and follows the rules that regulators set, and each member country keeps authority over its own numbers. The plan conforms with ITU Recommendation E.164, which NANPA calls the international standard for telephone numbering plans. That common standard is what lets a call be addressed from one country's plan to another's.

A number is an address on the network. It doesn't belong to a particular wire or device, and that is easiest to see when a line is replaced. Atlantech Online, a business phone and fiber provider, replaces copper phone lines with a managed appliance that connects over broadband and 4G LTE. In its description of the cutover, the existing numbers for the fire panel, elevator, alarm panel and fax lines are ported to the new service. Atlantech says the central monitoring station keeps receiving calls from the same caller ID. The analog line has been replaced, and the rest of the public network still finds each device at the same number.

A call on the PSTN has two parts. Setup comes first: the carriers' equipment exchanges signaling messages, and the dialed number in them routes the call toward its destination. Once the far phone answers, the two ends share a voice path for the rest of the call.

On older parts of the network that path is a reserved digital circuit. On newer parts the voice travels as IP packets. The FCC's 2025 notice on IP interconnection describes IP voice networks that carry calls from end to end, and a handoff through a media gateway wherever a call has to cross circuit equipment. The caller sees none of this.

You reach the PSTN by buying access from a carrier, and that access now comes in several forms.

The original form is the analog line on a copper pair, known as POTS. A mobile subscription is another, and businesses have more choices. Atlantech's business phone service offers a cloud PBX, or delivery to a PBX the customer already owns over SIP, PRI or analog lines.

Microsoft Teams needs the same access for calls to public phone numbers. Microsoft's Teams Phone overview says a PSTN connection is separate from the Teams Phone license. The connection is what provides phone numbers and access to domestic, international and emergency calling.

In each case the carrier supplies the same two things, a public number and a route to other public numbers.

In the UK, PSTN is also the name of a legacy analog network. Openreach, a UK network operator, confirmed in September 2026 that it closes on January 31, 2027, and estimated that about 1.2 million lines were still on it. What closes is old equipment and old lines.

Customers can keep a landline by moving to a replacement service. The UK government's guidance on the change says providers are upgrading landlines to digital technology that uses an internet connection, and that nobody has to lose their landline service because of it.

In the wider sense used here, the PSTN carries on while carriers replace the technology inside it. Phone numbers and the connections between carriers stay in place as the equipment changes.