Managing a Successful PSTN Transition

The PSTN is the decades-old copper network that's been powering landline and analogue services in the UK. BT Openreach has already stopped selling PSTN and ISDN services, and the full ISDN switch off alongside the PSTN switch off was originally planned for December 2025.

That deadline may shift slightly, but many regional exchanges have already shut down. Salisbury, for instance, was one of the first. The switch is already underway, even if you haven't felt it yet.

Why now? According to BT, running the copper network accounts for a surprising 3% of the UK's total energy usage. Shutting it down is part of a national effort to modernise and reduce the country's carbon footprint.

As Robert Brown, Business Information Security Officer at FourNet put it, “The copper network is often cited as accounting for as much as 3% of the UK's energy usage. Even if the exact figure varies, what's clear is that it consumes a disproportionately large amount of power for ageing technology. Replacing it with more efficient, digital alternatives will have a meaningful impact on the UK's carbon footprint."

Most organisations aren't starting from scratch as they already have digital infrastructure in place. But the first step in any successful PSTN switch off migration is understanding what you still have running on analogue (Copper phone systems).

That means doing a proper inventory check:

• Look at your billing records 
• Talk to your facilities teams
• Walk the floors if you have to

This might sound like a hassle, but you might be surprised by what's still active--and you could uncover potential savings right away.

One of our engineers recently worked with a Central Government organisation that only discovered a remote security outpost, not connected to their on-premise phone system that was still running on copper services even after their PSTN replacement project had already begun. "The security guard couldn't make or receive calls," they said. "It had been completely forgotten because they were not already connected to their on-premise system and instead had their own BT analogue service delivered directly to the post."

And that's not a one-off. We're still seeing organisations relying on fax machines for high-value transactions and lift phones wired into analogue lines buried in walls since the 1980s.

"Another large department had no idea what analogue services they still had scattered across sites. They're only just going through that discovery process now. Fax operates differently from standard telephone calls and doesn't always work over every SIP provider. This is often overlooked if you assume faxes will work as before--it's essential to understand the protocol ('language') the fax uses and ensure your SIP solution supports it before migrating."

Replacing Emergency Lines During the PSTN Switch Off with Resilient Alternatives

Robert Brown emphasizes that it's not just the everyday business phone lines you need to consider the emergency lines affected by the PSTN and ISDN switch off are equally critical. These often-hidden systems can include lift phones, Redcare lines, alarms, building intercoms, and emergency corridor phones. Whilst general analogue are ordinarily connected to an on-premise phone system, emergency lines are often separate and have their own exchange-provided lines directly to the telephone, this can make it difficult to consolidate information and manage a transition effectively.

In one example, a financial sector leader discovered they couldn't easily migrate their emergency lines to IP because the cabling in some heritage buildings wasn't voice-capable. Replacing it would have required major structural work. This is why it's crucial to identify and address these challenges early, giving yourself time to implement the necessary changes without disruption.

Tracing and Locating All Phone Lines Before PSTN Migration

Discovery can often be the most difficult part. Unlike IP devices, analogue lines don't send out a heartbeat or signal, which means that you have to manually trace them. As one of our engineers put it, "It's sometimes pretty much a case of plugging a toner in and walking the building. Painful, but necessary."

And even when you know where everything is, the physical environment poses its own hurdles. "Lift shafts often block mobile signals, which can make GSM-based emergency phones unreliable. To solve this, organisations typically install specialised antennas or signal repeaters inside the shaft, ensuring reliable coverage from top to bottom."

The good news is that you don't have to rip and replace everything. Most organisations have options, and the right PSTN replacement depends on how much resilience you need.

SIP Trunks are the most common PSTN replacement route. Instead of routing calls over copper, they travel over your existing internet connection. It's cheaper, more flexible, and much easier to scale; and you can still use your existing numbers.

But what about legacy endpoints during the PSTN switch off? That's where gateways come in.Devices like AudioCodes let you plug old analogue devices into a digital network. We often use this method to bridge lift phones, door entry systems or lone analogue lines that would be expensive to replace.

“We mimic the BT lines on this device, and then deliver the service via SIP. No need to replace all the internal cabling in most cases.”

For organisations looking to modernise fully after the analogue switch off, cloud platforms like Microsoft Teams or hosted telephony services offer a way to unify voice, video and messaging without needing on-premise hardware.

There's no avoiding the PSTN switch off; but preparing early gives you control over how and when it happens. And in many cases, it saves money too.

Consolidating multiple ISDN or analogue lines into a single SIP Trunk dramatically cuts costs. You only pay for the channels you use, and scaling up or down is easy. There are also savings on maintenance, fewer site visits, and less dependency on physical infrastructure.

It also makes your communications future-ready. Want to move offices or set up a new site? With SIP, your number goes with you. Want to integrate calls into Microsoft Teams or your contact centre? That is much simpler too.

Moving away from analogue lines as part of the analogue switch off will also help reduce your carbon footprint. Transitioning away from copper helps reduce the energy burden of the UK's telecoms footprint. As we mentioned earlier, switching off the copper network will drastically reduce national energy usage.

BT Openreach has already stopped selling new PSTN and ISDN services – the first stage of the nationwide PSTN and ISDN switch off  in theUK.

The full nationwide PSTN and ISDN switch off was originally planned for December 2025 but was pushed back to January 2027, however, some areas have already been migrated, with exchange closures rolling out region by region. You can check your region's status via Openreach's All IP tracker.

That date may seem a long time off, but many services will be unreachable well before then and as we mentioned earlier, unforeseen issues can extend the PSTN migration timeline dramatically.

Robert Brown raised that “many people didn't realise that the PSTN switch off started two or three years ago. They're [BT Openreach] doing it region by region, and some exchanges have already migrated.“

Here's what a typical PSTN migration journey looks like as part of the UK's PSTN switch off:

1. Start with discovery. Gather billing data, check physical sites, trace analogue lines.
2. Assess what's critical. Emergency lines, alarm systems, gate entry, etc.
3. Choose your solution. SIP, GSM, cloud telephony, or hybrid.
4. Test and pilot. Especially for critical systems.
5. Go live in phases. Don't leave everything until the deadline.

You can download our full checklist here to guide your team through every step.

If you're not sure what services are still running on PSTN ahead of the PSTN switch off UK, you're not alone – we are supporting many organisations in the same boat.

But waiting increases the risk of downtime and the later you leave it, the harder it becomes to source engineers, schedule PSTN migrations and plan around critical dates.

At FourNet, we've helped Central Government departments, Health and Finance Services, and Local Authorities through their PSTN switch off migration. We offer audits, consultancy, and ready-made PSTN replacement solutions to bridge your legacy systems with tomorrow's infrastructure.

“Once this deadline hits, you either need to be switched, or you risk losing service. It really is that black and white.”

Get in touch today and let's make sure your transition is on track.