ISDN Switch-Off: What it means and what to do
If your business relies on ISDN for calls, conferencing or your phone system, the clock is ticking. ISDN (Integrated Services Digital Network) is being permanently switched off across the UK on 31 January 2027. That’s a hard, confirmed date. After that point, any business still running on ISDN will lose service entirely.
The good news is that moving off ISDN is simpler than most businesses expect. Getting it sorted now means you avoid the rush, the risk and the disruption that comes with leaving it to the last minute.
This guide explains exactly what the ISDN switch-off means, which services are affected, the straightforward steps you need to take, and answers the questions UK businesses ask most about making the move.
What is ISDN, and why is it being switched off?
ISDN is a type of digital phone line that has been the backbone of business telephony in the UK for decades. It allowed businesses to carry voice calls and data over the same lines. That was a significant step forward when it was introduced, but it’s now well past its prime.
The technology underpinning ISDN, and the wider PSTN (Public Switched Telephone Network) it sits within, is ageing infrastructure that BT and Openreach can no longer justify maintaining. Modern internet-based alternatives, particularly Voice over IP (VoIP) and SIP trunking, deliver better call quality, more flexibility and lower cost.
BT has confirmed 31 January 2027 as the date the network will close. Openreach stopped selling new ISDN lines on 5 September 2023, so if you’re still on ISDN, migration is the only path forward. ISDN switch-off and PSTN switch-off are often talked about as the same thing, and in practice they are: ISDN is one of the services running on the PSTN network that BT is retiring.
Which ISDN and Featureline services will stop working?
ISDN comes in several forms, and all of them will cease to function on the switch-off date. Unlike some services that will be automatically migrated by BT under their Protective Migrations programme, ISDN services require action from you.
The following services will stop working completely on 31 January 2027:
• ISDN2 (also known as Basic Rate Interface / BRI): commonly used by small businesses for 2-channel digital phone lines
• ISDN30 (also known as Primary Rate Interface / PRI): used by larger businesses requiring multiple simultaneous calls over a single connection
• BT Featureline and Featureline Multiline: ISDN-based business phone lines with call management features; these will cease on the switch-off date
• ISDN-dependent PBX systems: if your internal phone system (PBX) connects to the network via ISDN, it will stop working unless you migrate the line or replace the system
• ISDN-based broadband (ADSL where delivered over ISDN circuits): some legacy broadband connections that rely on ISDN infrastructure will also be affected
If you’re unsure whether your current phone system or broadband relies on ISDN, the easiest way to check is to look at your current provider bills or call your provider. If the words ISDN, BRI, PRI, Featureline or “digital lines” appear, you’re almost certainly affected.
What happens if you do nothing?
This is the critical question, and the answer is straightforward. If you don’t migrate before 31 January 2027, your phone lines will stop working. No warning calls. No temporary extension. The network closes, and services that haven’t moved, stop.
For many businesses, the phone isn’t just a communication tool. It’s a commercial lifeline. Missed calls mean missed customers. Extended downtime means lost revenue and damaged reputation.
There’s also a practical capacity risk. As the deadline draws closer, every provider, including us, will face increasing demand from businesses that have left migration late. Starting now means you get a planned, engineer-supported migration. Leaving it until late 2026 means joining a queue.
What does migrating from ISDN actually involve?
The good news is that migrating from ISDN to a modern VoIP or SIP solution is well-trodden ground. We handle it every day. Here’s how a straightforward migration works:
1. Audit. We start with a short conversation to understand what you currently have: how many lines, what number or numbers you want to keep, what your phone system looks like and how your team uses it. This typically takes 30 minutes.
2. Quote. Based on your audit, we provide a clear, itemised quote for your new VoIP or SIP trunk solution. No hidden fees. We’ll also show you how your costs compare to what you’re paying now.
3. Number port. Your existing number or numbers move with you. We manage the porting process directly. You don’t need to contact your current provider or manage any of the technical transfer. Numbers are protected throughout.
4. Configure. We set up your new service, configure your call management settings and test everything before your go-live date. If you have a PBX or call system, we advise on compatibility and next steps.
5. Go live. Your new service activates, your old line closes, and your business carries on without interruption. An engineer is on call throughout your switch.
What should you move to?
There are two main alternatives to ISDN, and the right one depends on the size of your business and how your phone system is set up.
Hosted VoIP
Hosted VoIP (Voice over Internet Protocol) delivers your phone calls over your existing broadband connection. It’s ideal for smaller businesses, sole traders, and anyone who wants a simple, flexible system without dedicated hardware. You get a full business phone number, call management features, and the ability to answer calls anywhere, all managed through an online portal.
SIP trunking
SIP trunking is the enterprise-grade alternative, designed for businesses that already have a PBX phone system and want to keep it. Rather than replacing your system, SIP trunks replace the ISDN lines feeding it, connecting your existing hardware directly to the IP network. It’s the like-for-like replacement that gives you all the capacity of ISDN at a fraction of the running cost.
Not sure which is right for you? That’s exactly what our free consultation is for.
Why TTNC for your ISDN migration?
We’re not a reseller or a broker. TTNC is a UK network operator. We own and operate our own platform, which means we have direct control over your migration from start to finish. No third-party delays, no handoffs, no excuses.
• Over 7 million numbers in our inventory: we’ll find the right number for your business, or port the one you already have
• Rated #1 UK telecoms provider on Trustpilot at time of review: nearly 1,000 reviews from businesses just like yours
• UK-based support team: real people, named on reviews, available when you need them
• Ofcom regulated and ISO 9001 accredited
• No contracts on standard products, no setup fees, no surprises on your first bill
The businesses that migrate smoothly are the ones that plan ahead. The ones that get caught out are those who assume January 2027 is still a long way off. It isn’t.
ISDN switch-off: frequently asked questions
Is ISDN the same as PSTN?
Not exactly, but they’re closely linked. PSTN (Public Switched Telephone Network) is the wider UK telephone network, and ISDN is one of the digital line services that runs on it. Both are being retired on the same date: 31 January 2027, as part of BT’s network closure.
When exactly does ISDN switch off?
BT has confirmed the ISDN and wider PSTN network will close on 31 January 2027. Openreach already stopped selling new ISDN lines on 5 September 2023, so anyone still on ISDN today is using a line that can’t be renewed or replaced like-for-like.
What do I replace ISDN with?
Most businesses move to either Hosted VoIP, which runs over your existing broadband and suits smaller teams, or SIP trunking, which connects an existing PBX system to the IP network and suits larger or more complex setups. We’ll recommend the right fit once we understand how your business uses its lines.
Will I lose my business phone number if I migrate?
No. Number porting is a standard part of every ISDN migration we handle. Your existing number or numbers move across with you, and we manage the entire porting process on your behalf.
How long does an ISDN migration take?
Most migrations complete within a single working day once your audit and quote are agreed, with an engineer on call throughout the switch. The audit conversation itself typically takes around 30 minutes.
What if I leave it until closer to January 2027?
You can still migrate, but you’re likely to be competing with a much larger volume of businesses for provider capacity in the final months before switch-off. Migrating earlier means a planned, unhurried switch rather than a rushed one.
Get your free ISDN migration quote
Tell us what you have today and we’ll show you exactly what you need tomorrow. A conversation is all it takes to get a clear plan, a confirmed quote, and a migration date that works around your business.
