Source: Openreach, 30 October 2026 Your provider may move you earlier; check your own dates.
What changes
- ISDN lines into your phone system stop working and need replacing, usually with a SIP trunk.
- Your numbers can normally be kept (ported) to the new service.
- Older systems may need a gateway or an upgrade to connect to SIP.
What people forget
Phone systems aren't the only things on copper lines. Lift emergency phones, alarms, door entry, card machines and franking machines may all depend on them. Use our analogue dependency checker to find them.
Is this the moment to replace?
If your system is also near the end of its support life, it may make sense to decide both at once. See IP Office 500 V2: support status and options for UK owners and Repair, upgrade or replace your IP Office?.
What you'll need to replace ISDN
Most phone systems on ISDN move to a SIP trunk, which needs an internet connection that can carry your calls. Some older systems need a gateway to convert SIP to the ISDN connection they expect. Others need an upgrade. Your maintainer can tell you which applies, and your provider can size the connection for your peak number of calls. See SIP trunks explained.
A sensible order of work
- List every phone line and what it's used for. Use the dependency checker.
- Ask your provider when your lines will move.
- Ask your maintainer whether your system supports SIP.
- Check your internet connection can carry your calls.
- Agree a porting plan for every number.
- Book the cut-over for a quiet time, with a fallback plan.
Common surprises
| Surprise | How to avoid it |
|---|---|
| A lift or alarm line stops working | Contact those suppliers early |
| A number didn't get ported | Check the full number list before signing |
| Calls break up after the move | Check bandwidth and router voice settings |
| Out-of-hours routing changed | Test day and night routing after cut-over |
| Emergency address wrong | Confirm the address held for each number you send out, and check 999 and 112 with your provider |
Is now the time to replace the system?
If your phone system is also near the end of its support life, the switch-off is a natural moment to decide both together. Moving lines and replacing the system in one project can save effort, but it also adds risk. Compare both routes in Repair, upgrade or replace your IP Office?, and see IP Office 500 V2: support status and options for UK owners if you run an older IP Office.
Common questions
When is the ISDN switch-off in the UK?
The old network closes on 31 January 2027, but providers may move you earlier. Check your own dates with your provider.
Will my phone system stop working when ISDN is switched off?
The system itself doesn't stop, but it loses outside calls on those lines until it's connected another way, through SIP trunks or an ISDN-to-SIP gateway. Whether your model can do that needs checking with your maintainer.
Can I keep my phone numbers after the ISDN switch-off?
Usually yes: numbers can normally be ported to a new service.
What else uses ISDN or analogue lines?
Lift emergency phones, alarms, door entry, card machines, franking machines and fax. Check each with its supplier.
Is BT switching off ISDN?
Yes. Openreach, which runs the network used by BT and most other providers, is withdrawing ISDN and analogue services. In Hull and some nearby areas KCOM runs the network and has its own plans, and a few other networks do too. Ask your provider. Your own provider will confirm when your lines move.
Is ISDN and PSTN the same switch-off?
Yes, they're part of the same programme. The PSTN is the traditional phone network, and ISDN runs on it, so both are being withdrawn.
Sources
Last checked 9 November 2026 against the sources listed here. Menus and defaults can differ by software release, so confirm them on your own system. Spotted a mistake? Tell us.
