What is PSTN (Public Switched Telephone Network)?

What PSTN Means

PSTN stands for Public Switched Telephone Network. It's the name given to the traditional global telephone system that connected the world for more than a hundred years -- the one built on copper wires, analogue signalling, and physical telephone exchanges. When you hear someone say "landline" or "the old phone network," they're usually talking about PSTN.

The "switched" part is where the old tech shows. A PSTN call is circuit-switched -- meaning that when you dial a number, the exchange physically opens a dedicated electrical path between you and the person you're calling, and that path stays open until one of you hangs up. For the duration of the call, you've got a private connection through the network. It's simple, it's reliable, and it was the engineering breakthrough that made mass telephony possible in the early 1900s.

The opposite approach is packet-switched, where voice is chopped into digital packets and sent across a shared network -- which is how VoIP works. Circuit switching dedicates a line to each call. Packet switching shares the network across thousands of calls at once. Packet switching is cheaper to run, easier to scale, and far more flexible, which is why the world has moved away from PSTN.

PSTN vs VoIP

The two systems do the same job -- connecting two people in a voice conversation -- but they do it in very different ways.

PSTN runs on dedicated copper lines and purpose-built telephone exchanges. It's analogue at the edges, though the core has been digital for decades. It carries one call per line and is tightly regulated.

VoIP -- voice over internet protocol -- runs on any IP network. It's fully digital from end to end. It packs multiple simultaneous calls onto a single broadband connection. It supports video, screen sharing, click-to-dial, and integration with CRM systems and contact centre software. It's also vastly cheaper to run at scale, which is the main reason telcos everywhere are retiring PSTN.

From a caller's point of view, the experience is usually indistinguishable -- you dial a number, someone answers, you have a conversation. What changes is what happens under the bonnet and what's possible around that conversation.

Why This Matters for Contact Centres

Contact centres have been hybrid environments for a long time. Many of them run modern SIP-based telephony for their internal systems but still connect to the PSTN at the edge, because customers call in on traditional phone numbers from traditional phone lines. The centre's PBX or cloud telephony platform handles the translation between the two worlds.

The reason this matters for payment compliance is that PCI DSS doesn't care whether the call came in over PSTN or VoIP. If card data enters the call audio, the whole environment is in scope. Modern phone payment tools like DTMF masking work on the audio stream itself, so they can protect a call regardless of whether the customer is on a PSTN landline, a mobile, or an internet-based phone. What matters is what happens to the tones, not where they came from.

The UK PSTN Switch-Off

The UK's PSTN is being retired. BT has announced a plan to shut down the legacy voice network and move every remaining service to digital alternatives. The original target was December 2025, then pushed back to January 2027 after concerns about vulnerable customers who rely on analogue devices. That 2027 date is the current published timeline for the full PSTN retirement.

The practical effect for consumers is that old-style home phones plugged directly into the wall socket will stop working as-is. Customers will either get a new handset that plugs into their broadband router, or their existing handset will be connected via an adapter. The phone number stays the same. The call quality generally goes up. The main risks are for people who rely on devices that piggyback on the phone line -- telecare alarms, burglar alarms, lift emergency phones, some payment terminals -- because those devices weren't designed for a digital line and might need replacing.

For businesses, the switch-off is bigger. Any company still running an ISDN or PSTN-connected PBX has to migrate to a SIP-based phone system before the deadline. Those that wait until the last minute are going to find installers fully booked and migration costs rising.

What PSTN Retirement Means for Phone Payments

If your contact centre is still on PSTN or ISDN lines, the move to VoIP is an opportunity, not just a cost. Modern SIP telephony gives you features that simply weren't possible on the old network -- programmable routing, real-time analytics, CRM integration, and direct compatibility with cloud-based payment tools.

On the payment side specifically, moving to SIP makes it far easier to plug in a DTMF masking service. Rather than needing specialist hardware to sit between your PBX and the PSTN, a cloud-based payment tool can join the call as a SIP participant, handle the card entry portion, and drop out again -- all transparent to the caller and the agent. That's the model we use at Paytia.

If you're planning a switch-off migration and you're also taking card payments over the phone, it's the right moment to rethink the payment flow at the same time. Our guide on how to take a payment over the phone covers the agent workflow, the PCI implications, and the questions to ask a telephony supplier before you sign a contract.

Will PSTN Still Exist Anywhere?

Yes, but not for much longer in the UK. Some other countries are moving on a slower timeline, and some specialist industrial and maritime use cases will keep legacy analogue voice circuits for years. For mainstream consumer and business use, though, the direction is clear: PSTN is being turned off and replaced with digital voice carried over broadband or dedicated IP networks.

The BT retirement plan is publicly announced and applies to BT Openreach infrastructure, which underpins most of the UK's fixed-line voice services. Other providers that rely on Openreach wholesale lines are following the same timeline because the underlying copper network is being switched off beneath them.