What is PSTN?
PSTN, the public switched telephone network, is the worldwide collection of circuit-switched telephone lines, fiber-optic cables, cellular networks, and switching centers that connects any two phones for a voice call. It is the traditional landline system most people grew up with, sometimes called POTS (plain old telephone service).
When you dial a number and a dedicated path opens between your phone and the person you are calling, that is the PSTN at work. It has carried voice traffic since the late 19th century and still underpins much of the world's calling.
Originally analog, the PSTN is now almost entirely digital at its core, even though the "last mile" to many homes and businesses still runs on copper as part of the broader underlying telephony infrastructure.
Why PSTN Matters
For support and contact-center teams, the PSTN is the reason customers can still reach a 1-800 number from any phone, anywhere, without an app or internet connection. Universal reachability is its biggest asset.
It also matters because it is being retired. Carriers worldwide are sunsetting copper lines (the UK targets a full shutdown by 2027, and US carriers are migrating off TDM), pushing contact centers toward VoIP and SIP. Teams running legacy phone trees face rising costs and shrinking support, which is why many are looking to replace legacy IVR systems.
Knowing what runs on PSTN versus IP helps CX leaders plan migrations without dropping call quality or losing reach.
How PSTN Works
The PSTN uses circuit switching: for the length of a call, the network reserves a dedicated end-to-end channel, guaranteeing consistent quality but using capacity inefficiently. That differs from packet-switched VoIP, which breaks audio into packets sent over shared IP networks. In computer-networking terms, PSTN is the classic example of a circuit-switched network.
Calls route through local exchanges and tandem switches, with the SS7 signaling protocol handling setup, routing, and teardown behind the scenes. In-call inputs use touch-tone DTMF signals, the tones an IVR listens for when you "press 1 for billing."
Modern AI systems connect to the PSTN through a SIP trunk and a gateway that converts between circuit-switched and IP audio, so a cloud agent can answer a normal phone call. This is the bridge behind voice agents wired into CRM and telephony.
How Fini Approaches PSTN
Fini's voice AI answers real phone calls that arrive over the PSTN through SIP connectivity, then applies the same reasoning-first architecture that powers its chat and email agents, resolving inquiries at 98% accuracy with zero hallucinations. PII Shield redacts sensitive caller data in real time, so card numbers spoken on a recorded line never land in storage.
Because deployment takes about 48 hours, teams can point a support line at an AI agent and join the shift toward modern voice AI platforms without disrupting their existing carrier setup. Book a demo to hear it handle a live call.
What does PSTN mean?
PSTN stands for public switched telephone network. It is the global, interconnected system of phone lines, exchanges, switches, and cables that lets any phone call any other phone. People also call it the landline network, POTS, or simply "the phone system." It carries traditional voice calls using circuit switching rather than the internet.
What is the PSTN in computer networks?
In computer networking, the PSTN is the textbook example of a circuit-switched network. For each call it reserves a dedicated physical path between two endpoints for the full conversation. That contrasts with packet-switched networks like the internet, where data is split into packets and routed independently. Early dial-up modems used the PSTN to carry data before broadband.
What is a PSTN phone?
A PSTN phone is any device connected to the public switched telephone network through a traditional landline, such as a corded home phone or an analog office handset. It works without internet access and uses a phone number routed through carrier exchanges. Mobile and VoIP phones can also reach PSTN numbers through gateways that bridge the networks.
Is the PSTN the same as a landline?
A landline is a phone physically wired into the PSTN, so the two are closely related but not identical. The PSTN is the whole network of lines, switches, and exchanges; a landline is one endpoint connected to it. Mobile and VoIP calls also traverse the PSTN at some point, but they are not landlines.
What is the difference between PSTN and VoIP?
The PSTN uses circuit switching over dedicated copper or fiber lines, while VoIP sends voice as data packets over the internet. PSTN offers reliable reach without internet access; VoIP is cheaper and more flexible. Most AI voice tools, including Fini, connect to PSTN numbers through SIP gateways so they can answer ordinary inbound calls.
Is the PSTN being phased out?
Yes. Carriers are retiring legacy copper PSTN infrastructure in favor of IP-based networks. The UK plans to switch off its PSTN by 2027, and US providers are steadily migrating off older TDM systems. Calls increasingly move over VoIP and SIP, which is why Fini and similar platforms connect to phone lines through internet-based trunks rather than copper.