Question 1

Is PSTN secure, and can PSTN calls be encrypted?

Accepted Answer

A PSTN call travels as unencrypted pulse code modulation, so it is readable by anyone with access to the copper pair, the switch that serves it, or the gateway where the call enters the circuit-switched network, and US carriers are required by 47 U.S.C. 1002 to keep the ability to isolate a subscriber's calls and deliver them to the government under lawful authorization.[3][11][1] Calls can be encrypted, but only by devices at both ends holding matching keys, such as the NSA-keyed STU-III and its Secure Terminal Equipment successor, because the network supplies no encryption and expects none.[12][13] Encryption applied on an IP leg does not survive the crossing into the PSTN: a media gateway has to convert the media into a circuit bearer channel, which requires the decoded audio, so an SRTP-protected VoIP call is in the clear from the gateway onward.[6][5] CALEA draws that same line, excusing a carrier from decrypting anything it did not supply the encryption for and hold the keys to.[1]

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

The PSTN still carries calls in every market checked for this answer, and whether it has an end date depends on who has the power to set one. The United States has no national switch-off: the FCC counted 15.0 million retail switched access lines in service on June 30, 2025, and each carrier retires its own network through discontinuance applications under 47 CFR 63.71, with AT&T telling investors it expects to stop providing copper-based services across the vast majority of its footprint by the end of 2029 [3][5][7]. The United Kingdom does have a single date, because BT Group is retiring the PSTN by 31 January 2027 and over two thirds of UK landlines had already moved to VoIP when the government published its migration guidance in May 2025 [1][2]. Deutsche Telekom finished moving its own German fixed-network lines to IP at the end of 2020, and Australia disconnects legacy fixed-line services 18 months after the nbn is switched on in an area, so most published dates cover one company's network rather than a country [9][11].

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]