The way UK businesses connect is about to change forever. BT has confirmed that both the PSTN (Public Switched Telephone Network) and ISDN (Integrated Services Digital Network) will be permanently retired. Originally scheduled for December 2025, the final deadline for the BT ISDN switch off has now been extended to January 2027 – but the impact remains the same.
This isn’t just a technical upgrade. The PSTN switch off UK 2027 affects every organisation still relying on analogue or digital ISDN lines, whether for voice calls, fax, payment terminals, alarms, or broadband access. In other words, if your business hasn’t yet prepared for the ISDN end of life UK, time is running out.
The legacy networks that powered business communications for decades are no longer fit for the demands of a digital-first economy. To stay connected, organisations must start planning their migration to modern IP-based solutions like VoIP, SIP, and hosted services. Those who delay risk service disruption, increased costs, and a rushed transition when the cut-off finally arrives.
In this guide, we’ll explain what the switch-off means for UK businesses, the risks of waiting, and the practical steps to ensure a smooth migration before 2027.
What Is Happening?
The UK’s communications infrastructure is undergoing its biggest change in decades. BT has announced that all legacy voice services running over the copper network — known as Wholesale Line Rental (WLR) — are being withdrawn. This includes the PSTN and a range of services built on top of it, such as ISDN2e, ISDN30e/PRI, and Centrex/Featureline.
The Public Switched Telephone Network (PSTN) is the analogue phone system that has underpinned UK business and residential telephony for generations. On top of this, ISDN (Integrated Services Digital Network) introduced digital lines, offering clearer voice calls and the ability to handle multiple connections. Smaller firms often used ISDN2e (two digital channels), while larger organisations deployed ISDN30e/PRI (up to 30 channels) for switchboards and call centres. Meanwhile, BT’s Centrex and Featureline services provided hosted telephony features delivered directly from the network.
All of these technologies, however, rely on copper wiring and hardware that is expensive to maintain and increasingly out of step with modern digital communications. That’s why BT has set a firm timeline for change. A nationwide stop-sell came into effect in September 2023, which means no new orders, line transfers, or feature changes are allowed on PSTN or ISDN services. The complete switch-off — the official BT ISDN switch off date — is scheduled for January 2027.
From that point forward, ISDN lines are being retired entirely, and businesses must adopt an ISDN replacement UK solution such as VoIP, SIP trunking, or hosted telephony. The transition is not optional: every organisation still running on PSTN or ISDN needs to plan its migration now to avoid disruption when the cutover arrives.
Why Is BT Switching Off ISDN & PSTN?
The retirement of ISDN and PSTN is not just a business decision by BT — it’s a technological necessity. The copper-based infrastructure that underpins these services is decades old, costly to maintain, and increasingly unreliable. As usage declines and digital demand grows, continuing to invest in legacy systems makes little sense.
Instead, the UK is moving to an all-IP communications environment, where voice is delivered over the internet using modern, flexible services. Businesses can now replace legacy PRI (ISDN30e) circuits with SIP trunks, or adopt fully hosted solutions such as cloud PBXs and hosted VoIP UK platforms. These alternatives offer the same functionality as traditional lines — often far more — but without the physical limitations of copper wiring.
The benefits of VoIP migration UK go beyond cost savings. IP-based systems are highly scalable, allowing businesses to add or remove users in minutes. They’re also designed for mobility, supporting remote workforces and hybrid teams without additional infrastructure. Perhaps most importantly, they integrate with wider business applications — from CRM and collaboration tools to contact centre platforms — delivering a unified communications experience that ISDN and PSTN simply cannot provide.
In short, the shift from SIP vs PRI is less about losing old services and more about gaining access to faster, smarter, and more flexible communications. The switch-off represents a chance for businesses to modernise and future-proof their communications before 2027.
Which Services Are Affected?
The PSTN switch-off will impact far more than just traditional phone lines. Any service that relies on the old copper network will be withdrawn by January 2027, meaning organisations must plan their PSTN replacement for business now.
For voice, this includes standard analogue lines used by small offices and legacy systems, along with ISDN2e and ISDN30e/PRI connections that provide multiple channels for larger businesses, switchboards, and call centres. BT’s hosted telephony products, such as Centrex and Featureline, are also being retired, making it essential for organisations still using them to evaluate a suitable BT Featureline replacement.
But the changes go beyond voice services. Many critical business functions still depend on analogue connectivity. Fax machines, alarm systems, payment terminals (PDQ machines), lift emergency lines, and door entry systems all use PSTN or ISDN in some form. Without a clear migration plan, these devices will stop working once the copper network is decommissioned. For example, companies using card machines will need to adopt IP-enabled payment solutions as their PDQ machines PSTN replacement, while security and alarm systems will require digital-ready alternatives.
Another major consideration is telephony hardware. Many businesses still operate legacy PBX systems that are designed to work only with ISDN connections. Once those ISDN circuits are withdrawn, these PBXs will no longer function unless they are adapted with SIP gateways or migrated entirely to an IP-based system such as hosted VoIP or cloud PBX. Organisations that delay may face rushed, costly upgrades if they fail to address this before the deadline.
In short, the move from analogue phone lines to digital will affect almost every aspect of business communications. Understanding which services are at risk now gives organisations the time to plan their transition, ensuring continuity across voice, payments, safety systems, and beyond.
What Businesses Need to Do Now
With the ISDN and PSTN switch-off drawing closer, UK organisations cannot afford to wait until the last minute. Every business must prepare for the transition to digital communications. The following steps provide a clear framework for action.
1. Audit your current lines
The first step is understanding what you have today. Many companies still don’t realise they are running services on legacy connections. Check if your organisation uses ISDN2e, ISDN30e/PRI, Centrex, Featureline, or analogue PSTN lines. Don’t forget to include supporting services such as fax machines, PDQ payment terminals, lift phones, and alarm lines.
2. Decide on your replacement strategy
Once you know what’s in place, you can explore ISDN alternatives for business:
- Hosted VoIP / Cloud PBX – Ideal for SMEs, these solutions remove the need for on-site equipment. Calls run over the internet, with features like voicemail-to-email, mobile apps, and easy scalability. This is the simplest way to migrate from ISDN to VoIP.
- SIP trunking – Best suited to larger organisations that want to retain their on-premises PBX. With SIP trunking vs PRI, SIP provides greater flexibility, resilience, and lower costs, without the restrictions of physical ISDN circuits. A PRI to SIP migration ensures continuity for existing telephony infrastructure while future-proofing connectivity.
- All-IP connectivity – To support these services, businesses may need to upgrade broadband lines. Fibre options such as FTTP (Fibre to the Premises) or SoGEA (Single Order Generic Ethernet Access) deliver the reliability and bandwidth required for high-quality voice and data.
3. Migrate supporting services
Don’t overlook non-voice services. Fax, alarm lines, payment terminals, and lift lines all need to be adapted for IP. Most suppliers now provide digital-ready options, but early planning is key to avoid service disruption.
4. Plan for the Centrex switch-off migration
Organisations using BT Centrex or Featureline will need to act quickly. These hosted legacy services will cease alongside ISDN, meaning a move to modern hosted VoIP or cloud PBX is the only viable option.
5. Work with a trusted telecoms partner
Finally, businesses should engage with a telecoms partner (like Plexatalk) that understands the challenges of migration. An experienced provider can map out a timeline, recommend the right solution, and ensure a smooth transition without downtime.
Common Questions About BT ISDN & PSTN Switch Off
Will my existing phone numbers be lost?
No. Your existing numbers can be ported during the ISDN replacement UK process, whether you move to SIP trunks or a hosted solution. This ensures continuity for customers and suppliers. Number portability is standard practice in all UK VoIP migrations.
What about broadband?
Yes, broadband is affected. ADSL and FTTC run on the PSTN copper network, which is being retired. As part of the PSTN migration to VoIP, businesses must upgrade to fibre-based services such as FTTP or SoGEA to maintain connectivity.
What’s the cost of migrating?
Migrating usually reduces costs long-term. While there may be upfront setup or hardware expenses, VoIP and hosted PBX replacement for Centrex services typically offer lower call charges, no line rental, and reduced maintenance. Most businesses see savings after migration.
Do I need new hardware or handsets?
Possibly. Legacy PBXs built for ISDN need either a SIP gateway or replacement with an IP-based system. Many organisations use the switch-off as an opportunity to upgrade to modern handsets, softphones, or cloud-based apps that work seamlessly with hosted VoIP platforms.
Next Steps & How to Prepare
The BT ISDN switch off and PSTN switch off UK 2027 may feel distant, but waiting until the deadline is risky. Businesses that delay could face service disruption, limited supplier availability, and a rushed, more expensive transition. Acting now ensures a smoother and more strategic migration.
A simple action plan can keep you ahead of the curve:
- Audit your lines and services – Identify every PSTN or ISDN dependency, from phone lines to alarms, faxes, and card machines.
- Choose the right solution – For most SMEs, hosted VoIP or cloud PBX offers flexibility and cost savings. Larger organisations may prefer SIP trunking for a phased approach.
- Plan your connectivity – Ensure you have the right broadband in place, such as FTTP or SoGEA, to support IP-based services.
- Migrate in stages – Start with non-critical services, then move core voice systems. Early migration gives time to test and adjust.
- Engage with an expert partner – Specialist providers can design a tailored roadmap for your VoIP migration UK, ensuring business continuity throughout.
The switch-off is not just about replacing old lines — it’s about upgrading to a communications platform that supports growth, remote working, and digital transformation. Don’t leave it until the last minute. Start planning today and future-proof your business.
Migrate Today
The ISDN end of life UK marks a turning point for every organisation still relying on analogue or digital lines. With the PSTN switch off UK finalised for January 2027, businesses cannot afford to ignore the change. The risks of waiting are clear: potential downtime, higher costs, and last-minute migrations that leave no room for careful planning.
The good news is that alternatives are already proven and widely available. Whether you choose SIP trunking to extend the life of your existing PBX or decide to migrate from ISDN to VoIP with a hosted cloud solution, the move will unlock flexibility, scalability, and cost efficiency that legacy services simply cannot match.
Now is the time to audit your systems, explore your options, and start your migration journey with confidence. By acting early, you can turn a mandatory change into a strategic upgrade that benefits your business long before the final deadline.
Need Help With Migration?
Don’t wait until 2027. Start preparing today and ensure your communications are ready for the future.