What is the difference between PSTN and POTS?

A POTS line is an access service. It runs from the telephone company's local office to a jack in your building, and the same pair of wires does several jobs.

It carries your voice as an analog electrical signal. It also carries the signaling. Cisco's guide to analog signaling describes the common loop-start arrangement. The telephone company's end supplies the current and the ringing signal. The phone asks for service by closing the circuit when the handset is lifted, and the office detects the current and returns a dial tone.

The line carries power as well. The UK government's guidance on the move to digital landlines says an analog landline brings a low-voltage power connection from the telephone exchange, enough to run some basic corded handsets. Those handsets keep working in a local power cut as long as the exchange still has power.

A lot of equipment was designed around those properties. A fire alarm panel, an elevator phone or a fax machine expects to find dial tone on a two-wire jack, and some of it expects the line to stay up when the building's power is off.

Your line is the last stretch of the public network. Behind it are the switching, the long-distance links and the connections between carriers that take a call from your telephone company to someone else's.

The public network is much wider than POTS. US rules, in 47 CFR 20.3, define the public switched network as common carrier switched networks, by wire or radio, that use the North American Numbering Plan. The definition names local exchange carriers, long-distance carriers and mobile providers. Under it, a call between two cell phones travels on the public switched network, and no POTS line is involved.

The PSTN has not been an analog network for a long time, either. On a POTS call the voice is analog along the copper pair, and a traditional circuit network then carries it in digital form. ITU-T G.711, the standard for that coding, samples speech 8,000 times a second and uses eight bits for each sample. That makes 64,000 bits per second.

Past that point, carriers increasingly use IP. In a 2025 notice on IP interconnection, the FCC describes voice networks that carry calls from end to end over managed IP cores. Where a call has to cross older circuit equipment, a media gateway converts it.

For decades, homes and small businesses reached the public network through POTS lines. The line and the network looked like one thing, and the habit of treating them as one has lasted.

It shows up in official language. The UK guidance describes "the retirement of analogue phone lines, also known as the Public Switched Telephone Network (PSTN)." In the UK, PSTN is the name of a specific legacy network. Openreach, a UK network operator, confirmed in September 2026 that this network closes on January 31, 2027. It estimated that about 1.2 million lines were still on it.

That closure ends analog lines on one operator's old network, while public telephone calling in the UK carries on. The same guidance says providers are upgrading landlines to digital technology that uses an internet connection, and that nobody has to lose their landline service because of the change. A customer who moves to the digital service still has a landline, and its calls still cross the carriers' interconnected networks.

US rules separate the wire from the service in a similar way. 47 CFR 51.325 defines copper retirement as removing or disabling copper loops, or replacing them with fiber to the home or to the curb. That is a change to the wire. A carrier can retire the copper and still deliver phone service to the same address over fiber.

The distinction matters most when a POTS line is being withdrawn. The device on that line still needs what the line gave it. It also still needs to reach the public network. Those are separate jobs, and a replacement has to do both.

Atlantech Online, a business phone and fiber provider, shows how the jobs divide in its POTS replacement service. A wall-mounted appliance in the telecom closet takes the place of the copper lines, and Atlantech says it gives every device the same analog dial tone it has always used. Each existing line is moved to an FXS port, the port type that plays the telephone company's role toward an analog device. One unit supports two to eight lines, and Atlantech puts the typical cost at $15 to $45 per line per month plus hardware.

The appliance does not need an analog line from the telephone company. Calls reach Atlantech's voice network over broadband, with 4G LTE as a second path. Power no longer arrives from the telephone company, so the unit has its own battery, which Atlantech rates at up to 24 hours.

The PSTN side barely changes. Atlantech says it ports the existing numbers for the fire panel, elevator, alarm panel and fax lines, so that a central monitoring station keeps receiving calls from the same caller ID. Its cutover process tests fire and elevator lines with the monitoring center after each line moves. The number was never a property of the copper. It is an address on the public network, and it moves with the service.

So a building can lose every POTS line it has and remain on the PSTN. What has to be checked, device by device, is whether each one works on the replacement line and how long it runs when the power is out.