Question 1

Is PSTN secure, and can PSTN calls be encrypted?

Accepted Answer

The public switched telephone network offers almost no built-in security, and an ordinary landline call travels across it with no encryption, so it can be overheard by anyone who taps the copper line, gains access inside a telephone exchange, or reaches the signaling that controls the call.[1][9][12] The standard that defines how the PSTN digitizes voice, ITU-T G.711, encodes speech as pulse code modulation and includes no encryption of its own, and lawful interception is a designed-in feature of carrier switches rather than an add-on.[2][1][6] The control network behind call setup, Signaling System No. 7, was built on the assumption that connected operators could be trusted, and researchers and a US cybersecurity official have documented its use to track people and intercept calls and texts.[3][4][5] A PSTN call can be encrypted, but only by adding matching cryptographic equipment at both ends, such as the NSA's STU-III and its successor STE, or by moving the conversation off the circuit-switched network onto an IP path protected by protocols like SRTP.[7][8][11] The network itself never supplies the encryption, so any confidentiality has to come from the endpoints.[9]

Question 2

Which legacy devices are still at risk when PSTN/POTS lines are retired or moved to VoIP?

Accepted Answer

The devices most likely to fail are the analog endpoints that dial out over a copper line and rely on its steady electrical signaling: fire alarm panel communicators, elevator emergency phones, security and burglar alarm dialers, medical alert pendants, and the fax machines and card-payment terminals that talk through analog modems.[10][3][4][6] Two problems trip them up on Voice over IP. The packet network compresses and occasionally drops the audio these devices treat as exact data, so signals arrive corrupted or never reach the monitoring center.[9][5] And a traditional copper loop drew its own power from the telephone exchange and kept working in a blackout, while a VoIP line goes dead in an outage unless it has its own battery backup.[2][11] Because carriers are retiring copper and shifting voice to IP, any of these devices needs a compatible replacement path, whether cellular, purpose-built IP, or a service engineered to emulate an analog line, before the line it depends on is cut.[1][11]

Question 3

What is a PSTN gateway, and what do FXS and FXO ports do?

Accepted Answer

A PSTN gateway is a device that converts telephone calls between the circuit-switched public telephone network and a packet-based IP network, so analog phones, fax machines, or a legacy PBX can work with a VoIP system.[1][2] On the analog side it uses two kinds of ports. An FXS port, short for Foreign Exchange Subscriber, supplies dial tone, battery current, and ring voltage to an end device such as a phone or fax machine, while an FXO port, short for Foreign Exchange Office, is the line-side interface that receives dial tone from the PSTN or a PBX and signals on-hook and off-hook by opening and closing the electrical loop.[4][11] The two always work as a pair, because a line coming from an FXO port has to terminate on an FXS port for a call to complete.[4] The gateway handles the media conversion, encoding voice with a codec such as G.711 or G.729 into RTP packets, and it speaks a VoIP control protocol such as SIP, MGCP, or H.248 to the phone system that runs the call.[1][3][8]

Question 4

What is the difference between PSTN and PBX?

Accepted Answer

The PSTN and a PBX operate at different scales. The public switched telephone network (PSTN) is the shared, carrier-operated system of interconnected telephone networks that can reach almost any phone in the world,[4][5] while a private branch exchange (PBX) is a single organization's own telephone switch that routes calls between its internal users and shares a small number of outside lines among them.[1] The PSTN belongs to national, regional, and local operators and works as public infrastructure,[5] whereas a PBX is privately owned and usually sits on a company's premises or, increasingly, in a provider's cloud.[1][2] The two are not competitors but layers that connect: a PBX links to the PSTN through trunk lines so its extensions can call the outside world, which is why an office needs far fewer external lines than it has phones.[1][3] Modern IP PBX and hosted PBX systems perform the same switching in software and often reach the PSTN over a SIP trunk instead of copper.[2][10]

Question 5

What is the difference between PSTN and POTS?

Accepted Answer

The difference is one of scope: the public switched telephone network, or PSTN, is the entire interconnected system of operator-run telephone networks, while plain old telephone service, or POTS, is the basic analog voice service delivered to a subscriber over that system.[1][2] POTS is therefore a service that runs on the PSTN, not a separate network, which is why the two names often get used for the same thing.[2][4] Technically, POTS is analog from end to end over a copper pair and limited to a narrow voice band of roughly 300 to 3,400 hertz, whereas the PSTN's core has been digital for decades, encoding each call as a 64 kbit/s stream.[1][3][6] Both are now being retired together as carriers move voice onto internet protocol networks, and the United Kingdom is scheduled to switch off its PSTN, and the POTS lines it carries, by 31 January 2027.[7][8]

Question 6

Is PSTN still used today?

Accepted Answer

Yes, the PSTN is still used in 2026, but it now carries a small and shrinking share of voice traffic and is being deliberately shut down in most developed markets. In the United States, the count of traditional switched access lines that make up the PSTN fell from about 181 million in 2003 to roughly 15 million by mid-2025, while interconnected VoIP grew to about 63 million subscriptions [3]. The United Kingdom plans to retire its PSTN entirely by 31 January 2027, and roughly two thirds of UK landlines have already moved to digital voice [1][2]. Some national networks, including Germany's and Australia's, have already completed or nearly completed the move to all-IP [6][7]. So the honest answer is that the PSTN still works and still carries live calls, yet it is a legacy network in a managed retirement rather than something being newly deployed [4].

Question 7

What is the difference between PSTN and VoIP?

Accepted Answer

The core difference is how each network moves a call. The public switched telephone network (PSTN) uses circuit switching, which reserves a dedicated path for the whole conversation, while Voice over Internet Protocol (VoIP) uses packet switching, breaking speech into data packets that share an IP network with your other traffic.[1][2][3] The PSTN runs over operator-owned copper and digital trunks and sets up calls with a separate signaling network called SS7, whereas VoIP runs over broadband and sets up calls with internet protocols such as SIP, carrying the audio in RTP packets.[1][5][6][7] Because VoIP treats voice as data, it usually costs less to run and can add features or capacity in software, though it now depends on your power and internet staying up.[2][9] The two are less rivals than successive generations: carriers are retiring the PSTN in favor of IP, and in the United Kingdom the switch off is scheduled for 31 January 2027.[2][10]

Question 8

How does the PSTN work?

Accepted Answer

The PSTN works by circuit switching: for each call it builds a dedicated end-to-end path between two phones and holds that path open until someone hangs up.[3] A call starts when you go off-hook and your local exchange, the central office that terminates your line, detects the loop current and returns a dial tone, then collects the digits you dial.[4] The exchange reads that number, which follows the ITU-T E.164 numbering plan, and either connects the call itself or routes it up through tandem and long-distance switches to the exchange serving the other phone.[9][1] A separate control network called Signaling System No. 7 carries the setup and teardown messages out of band, so the switches can ring the far end, connect the two parties, and release the circuit without tying up the voice path.[7][6] Through the digital core the conversation travels as a 64 kbit/s channel encoded with pulse code modulation and time-division multiplexed onto shared trunks.[8][10]

Question 9

What is the PSTN (Public Switched Telephone Network)?

Accepted Answer

The public switched telephone network, or PSTN, is the aggregate of the world's interconnected, operator-run telephone networks, and it has carried ordinary landline calls for well over a century.[1] It works by circuit switching, meaning the network reserves a dedicated end-to-end path for each call and holds it open for the whole conversation, which is the main way it differs from the packet networks that carry internet traffic.[2] A call travels from your phone over a local loop to a local exchange, then across shared trunks and higher-level switches to the exchange serving the person you are calling, with a separate control network called SS7 setting up and releasing the connection.[1][4] Since the 1960s the core of the network has been digital, encoding each voice channel as a 64 kbit/s stream and multiplexing many of them onto shared circuits.[3][7] Carriers are now retiring the PSTN in favor of internet protocol networks, and in the United Kingdom the switch off is scheduled for 31 January 2027.[8][9]