What is the difference between PSTN and POTS?

PSTN and POTS describe two different layers of the same telephone system: one is the network, the other is a service that runs on it. The public switched telephone network is the aggregate of the world's interconnected, operator-run telephone networks, the physical and logical system of exchanges and trunks, plus the switches that route a call between any two phones.[1] Plain old telephone service is the basic, two-way analog voice service that reaches a subscriber over a copper line, the ordinary landline most people grew up with.[2] Every POTS line depends on the PSTN to complete a call, but the PSTN carries much more than plain voice, so POTS is best understood as one service the network delivers rather than a rival to it.[1][2]

Why the two names blur together

For a home phone user, POTS and the PSTN arrive as the same thing. When you picked up an analog handset and heard a dial tone, you were using POTS, and the call it placed traveled across the PSTN.[2][4] Reference sources carry that overlap: some define POTS as the analog way of accessing the PSTN, and at least one industry glossary lists the PSTN as also being known as plain old telephone service.[1][4] The distinction still matters, because the PSTN also carries digital services, business trunks, and the signaling that plain telephone service never touched, and because the two are being retired on the same timeline as voice moves to internet protocol.[1][7]

Put simply, POTS is the service a customer buys on a single line, and the PSTN is the switched system that connects that line to every other phone. Keeping the two straight matters most during a migration, because retiring the analog service on a line is not the same event as rebuilding the network beneath it, even if the current transition happens to do both.[1][7]

The clearest way to separate the two is by scope: the PSTN is infrastructure, and POTS is a service that infrastructure provides. The public switched telephone network is the system of local loops, exchanges, trunks, and switches that any call travels through, owned not by one company but by many carriers interconnecting under shared standards.[1] Plain old telephone service is what a subscriber actually receives on their line: a dial tone and a two-way voice path that reaches any other number.[2] You can run the network without a given service on it, but you cannot have the service without the network, which is why POTS sits inside the PSTN rather than beside it.[1][2] For a business deciding what to replace, the practical takeaway is that dropping POTS lines changes the service on your premises, while the PSTN switch-off changes the carrier network those lines depend on.[5][7]

POTS is analog from the handset to the exchange, while the core of the PSTN has been digital for decades, and that split is the source of much of the confusion. A plain telephone line carries the voice as a continuously varying electrical signal over a copper pair, confined to a narrow band of roughly 300 to 3,400 hertz, which is enough for intelligible speech but well short of full-range audio.[3] Once that analog signal reaches the local exchange, the PSTN converts it into digital form, sampling the voice 8,000 times a second and coding each sample to produce a single 64 kbit/s channel defined by the ITU-T G.711 standard.[6][1] From there the call crosses the network as digital data and is turned back into analog only for the copper line at the far end.[1] So a normal landline call is analog at both edges and digital in the middle, and POTS names the analog edge while the PSTN names the whole path, most of which is no longer analog at all.[1][3]

The word switched in PSTN refers to circuit switching, the method the network uses to build a temporary dedicated path for each call, and POTS is the most basic use of that capability. When a POTS line places a call, the PSTN's switches read the dialed number and connect a path through a hierarchy of exchanges, holding that circuit open for the length of the conversation.[4] Plain telephone service adds nothing to this arrangement: it is a point-to-point voice line that depends on the network's switching to reach the other end, without a data channel or signaling features of its own.[2] The routing and the call setup live in the PSTN, not in the POTS line.[1][4]

POTS is bare voice; the PSTN carries more

Beyond that single voice path, POTS provides very little, while the PSTN also carries the signaling and numbering, plus the business services that plain telephone service never included. A POTS line is powered from the exchange, typically at about 48 volts of direct current backed by central-office batteries, which is why a wired analog phone usually kept working during a household power cut.[2] Features such as caller identification depend on the wider network rather than the line itself.[2] The PSTN also supports business trunks and the signaling that sets up calls, along with legacy digital services such as ISDN that emerged on the same infrastructure.[4][1] For a buyer, this is why a modern replacement has to account for more than dial tone: an old POTS line may quietly be running a life-safety circuit such as a fire panel or elevator phone that assumed the line would stay powered and available.[9][5]

People treat PSTN and POTS as synonyms because, for an ordinary landline, they describe the same experience from two different angles. To a residential caller there was never a visible boundary between the service and the network, since the analog line and the call it placed across the country felt like a single thing.[2][4] Technical writing has absorbed that habit, so you will find POTS defined as the analog way of accessing the PSTN, and you will also find the PSTN listed as being known as plain old telephone service.[1][4] The looseness is usually harmless, but it hides a real distinction: the PSTN carries services that are not POTS, including business-grade digital lines and the signaling that makes the whole system work, none of which a plain telephone line ever used.[1][4] In a migration plan the difference resurfaces, because a provider can retire the analog POTS product on your line while the underlying network keeps running, and it can also rebuild the network underneath you while your service appears unchanged.[5][7]

Retiring POTS and the PSTN means moving telephone calls off dedicated copper circuits and onto internet protocol networks, a change carriers are making because the old equipment is expensive to keep running.[5] The switch is happening on both sides of the Atlantic, though the timelines and the way they are framed differ.

The UK switch-off has a fixed date

In the United Kingdom the transition has a hard deadline: BT Group plans to retire the PSTN by 31 January 2027, after which landline calls run over broadband as Voice over IP.[7] Openreach, which operates the physical network most UK providers rely on, has been withdrawing the products that depend on the old network and has stopped selling new copper-based lines.[7] The UK government describes the analog network as having reached the end of its serviceable life and treats the move to digital landlines as an industry-led program expected to be substantially complete by the same date.[8] Because a digital line does not draw power from the exchange the way a POTS line did, providers must offer a way to keep reaching emergency services for at least an hour during a power cut, usually a battery backup unit.[8]

The US copper sunset runs on a looser timeline

In the United States the same shift is underway without a single national switch-off date. Carriers are transitioning the PSTN to all-IP, and most households have already moved to wireless or internet-based calling.[4][5] Regulators have cleared the path by relieving carriers of the obligation to maintain aging copper, so companies including AT&T and Verizon have been decommissioning copper networks and steering customers toward fiber and wireless.[5] One visible effect for businesses still on analog lines is price, since standalone POTS service has grown markedly more expensive as carriers add copper surcharges to encourage migration.[9][5]

The strengths and weaknesses of POTS and the PSTN both come from their age and their circuit-switched design. Reliability is the classic argument for POTS, because the line carries its own power from the exchange, so a basic analog phone usually kept working when the mains failed, which is exactly the property digital replacements have to recreate with backup batteries.[2][8] Voice quality and call setup are consistent too, since circuit switching reserves capacity for each call rather than sharing it the way packet networks do.[4]

The limits are just as real. A POTS line is confined to a narrow voice band, so it was never a path for anything beyond speech and slow dial-up data.[3] Holding a full circuit open for every call, including the silent gaps, uses network capacity less efficiently than internet protocol, which is part of why carriers prefer IP.[4][1] The maintenance cost is the deciding factor now, because the copper plant and the switches behind it are decades old and increasingly hard to support, so the timing of a migration is set more by the carrier's retirement schedule than by the customer's preference.[5][7]