BT ISDN Switch-Off: Dates, Deadlines and What to Do Now
Over 2 million UK businesses are still running phone systems that will stop working on 31 January 2027. BT's ISDN switch-off is not a distant concern — it is a confirmed deadline, less than two years away, with no further extensions expected. If your business uses ISDN lines, a traditional PSTN landline, or ADSL broadband, this is the event that makes your current setup obsolete. Here is what you need to know and what you need to do.
The short answer: BT and Openreach are switching off all ISDN and PSTN services in the UK on 31 January 2027. New ISDN orders stopped in September 2023 — if you are still on ISDN, you are on a discontinued product with a confirmed end date. The replacement is VoIP. Businesses that migrate now pay less per month immediately and avoid the backlog of last-minute migrations in late 2026.
The ISDN switch-off timeline
ISDN has been on borrowed time for nearly a decade. Here is the full timeline:
November 2017 BT and the wider UK telecoms industry formally announced the planned retirement of PSTN and ISDN networks. The industry began preparing wholesale customers and business users for the transition.
September 2023 Openreach stopped accepting new ISDN orders across the entire UK. If your existing ISDN lines are still running, they are on the last generation of circuits that will ever be installed.
January 2027 (original deadline) The original switch-off date was December 2025. BT and Openreach postponed it by thirteen months to allow more time for vulnerable users, safety-critical devices (personal alarms, telecare systems), and businesses with complex migration requirements to complete their transitions.
31 January 2027 The confirmed final date for the complete switch-off of all PSTN and ISDN services across the UK. All copper-based phone lines and ISDN circuits will cease to function on this date.
There is no indication of a further extension. The January 2027 date has been confirmed by Openreach, the UK government, and Ofcom. Planning around an extension is not a strategy.
Who is affected by the BT ISDN switch-off
The honest answer is: more businesses than realise it. According to industry estimates, over 2 million UK businesses are affected by the switch-off. Approximately 31% of UK businesses have already migrated to VoIP — which means 69% have not.
Your business is affected if you use any of the following:
ISDN2 or ISDN30 lines Standard ISDN services used for multi-line business phone systems, PBX connectivity, or high-capacity voice. All ISDN circuits will be withdrawn on 31 January 2027.
PSTN landlines (analogue copper) Traditional analogue phone lines — the standard "phone line" installed in UK offices since the 20th century. These run on the same copper network as ISDN and will be switched off simultaneously.
ADSL or ADSL2+ broadband ADSL broadband routes over the PSTN copper network. When the PSTN goes off, ADSL goes with it. If your office internet is ADSL, you need to upgrade to FTTC, FTTP, or a leased line before the deadline — not just your phone lines.
Devices that depend on a phone line PSTN-connected devices stop working at switch-off. These include:
- Monitored intruder alarms and fire detection systems
- Personal alarms and telecare devices (2.3 million people in the UK depend on these)
- Card payment PDQ terminals on older connections
- Door entry and intercom systems
- Fax machines
- Lift emergency phones
If your business uses any of these, contact the relevant supplier now, not in late 2026.
What happens if you do nothing
Your phones stop working on 31 January 2027. Not "degrade" or "become less reliable." Off. Permanently.
For most businesses, the consequences cascade:
- Customers cannot call you on any existing number
- You cannot make outbound calls to suppliers, banks, or partners
- Any PSTN-dependent system (alarms, payments, access control) stops functioning
- Your existing phone numbers are at risk if not ported in advance — you cannot port numbers from a service that has already been terminated
The businesses most likely to face disruption are those that delay the migration until late 2026. By that point, the UK's VoIP providers will be handling the backlog of every business that waited — installation timelines will be extended, support capacity stretched, and your ability to control the timing of your own migration significantly reduced.
The businesses migrating now get:
- Lower monthly costs immediately (ISDN costs £30–£50 per channel; VoIP costs £5.99–£24.99 per user)
- A controlled migration on their own schedule
- Time to test, configure, and train staff without a deadline forcing their hand
The five steps every ISDN business should take now
Step 1: Audit everything that runs on a phone line Get a complete list of every ISDN circuit, analogue line, and ADSL broadband connection your business uses. Include alarm systems, PDQ terminals, door entry systems, and any other device that connects via a phone socket. Your current BT Business bill is the starting point, but it may not capture everything — particularly alarm and security systems.
Step 2: Check your broadband type Log into your router or contact your ISP. If your internet connection is ADSL or ADSL2+, you need to upgrade it as part of your PSTN migration. FTTC and FTTP are the standard upgrades for most UK business addresses.
Step 3: Choose your replacement: hosted VoIP or SIP trunking Hosted VoIP replaces your phone system with a cloud-based service — no on-site server, modern features included as standard, from £5.99/user/month. SIP trunking replaces your ISDN circuits while keeping your existing on-site PBX, if it is SIP-compatible. For most SMEs migrating away from ISDN, hosted VoIP is the simpler and more cost-effective route.
Step 4: Port your numbers before cancelling anything Your existing ISDN numbers can be transferred to your new VoIP system via number porting. This takes 5–10 working days. Do not cancel your ISDN lines before porting is complete — if you do, you lose the numbers.
Step 5: Migrate and test before the deadline Do not schedule your migration for January 2027. Aim to be fully live at least 3–6 months before the deadline, leaving time to identify and fix any issues with call quality, device compatibility, or configuration. The closer to the deadline, the harder it will be to get support quickly if something goes wrong.
What ISDN actually costs versus VoIP
Most business owners are surprised when they work out what they are currently paying for ISDN. A typical ISDN setup looks like this:
| ISDN | VoIP equivalent | |
|---|---|---|
| Line rental per channel | £30–£50/month | — |
| 8-channel ISDN (typical 15-user office) | £240–£400/month in line rental alone | — |
| VoIP licence (15 users, Samurai plan) | — | £224.85/month |
| Call bundles | Included (limited) | 750 min/user/month included |
| Features | Basic | Call recording, voicemail-to-email, mobile app, ring groups, IVR included |
| Contract | 12–24 months, remaining fixed-line costs | 28-day rolling |
A 15-person business on ISDN is typically spending £300–£500+ per month in line rental alone. Moving to hosted VoIP at £14.99/user/month (Samurai) costs £224.85/month total — with more features and a 28-day rolling agreement instead of a long-term contract.
How VoIPninjas handles ISDN migrations
VoIPninjas is a direct UK provider — not a reseller — based in Christchurch, Dorset. We have been helping businesses across the South Coast and Channel Islands migrate from ISDN to VoIP, and we handle national migrations on the same terms.
What our ISDN migration includes:
- Free consultation: we review your ISDN setup, call volumes, and any PSTN-dependent devices — not just the phones
- Number porting: your existing numbers transfer to the new system before the old lines are cancelled
- Concurrent migration: setup, porting, and testing run simultaneously so total transition time is minimised
- Direct UK support: because we own our infrastructure, the person you call is the person who can fix the problem
Our hosted VoIP plans start at £5.99/user/month on Ronin (100 min/month) on a 28-day rolling agreement. Most ISDN migrations are complete within 10 working days.
You can start with a free 14-day trial — no card required — to verify call quality on your existing connection before committing to anything.
Start your ISDN migration — don't wait for the deadline No tie-in, no setup fees. Most ISDN migrations are complete within 10 working days. Get a Free Quote — or call us: 0330 043 2388
Frequently asked questions
When is the BT ISDN switch-off date?
31 January 2027 is the confirmed date for the complete switch-off of all ISDN and PSTN services in the UK. Openreach stopped accepting new ISDN orders in September 2023. The original December 2025 deadline was extended by thirteen months, but the January 2027 date is confirmed and no further extension is anticipated.
Will I lose my existing phone number when ISDN is switched off?
Only if you do not port your numbers before the switch-off. Number porting lets you move your existing business numbers to a new VoIP provider. The process takes 5–10 working days. Your VoIP provider manages the porting request, and your ISDN lines remain active until the port completes. If you wait until after the switch-off to try to port your numbers, the numbers may already be released and unrecoverable.
Is the ISDN switch-off just about phones?
No. The PSTN copper network supports over 300 types of devices and services beyond voice calls. Monitored alarm systems, personal alarms, CCTV monitoring, PDQ card payment terminals, door entry systems, and fax machines may all depend on PSTN connectivity. Each of these needs to be checked and migrated separately from the phone system itself.
My ISDN contract does not expire until after January 2027 — what happens?
Your provider should be in contact about early migration options. Most carriers are offering no-penalty early exits from ISDN contracts specifically because the service is being discontinued. If your provider is not proactively offering this, contact them directly and ask about early migration terms — you should not be penalised for migrating ahead of a forced switch-off.
Do I need a new broadband connection to use VoIP?
Not necessarily. VoIP works over FTTC, FTTP, or a leased line. If you currently have FTTC or FTTP broadband, you can run VoIP on it without any broadband changes. If your broadband is ADSL, you need to upgrade your internet connection as part of the PSTN migration — ADSL will go offline with the PSTN switch-off.
How long does switching from ISDN to VoIP take?
With VoIPninjas, most ISDN migrations are complete within 10 working days from the date you confirm your order. The main variable is number porting, which takes 5–10 working days and runs concurrently with the rest of the setup. We recommend starting at least 3–4 weeks before your preferred go-live date.