ISDN stops working on 31 January 2027. In 25 years of working on business phone systems, I've found it's rarely the phones that cause the trouble. It's everything wired into them that nobody wrote down.
Where we are
Openreach has confirmed there will be no further extensions.
What ISDN actually is
ISDN is the digital line that connects an on-site phone system — the box in the comms cupboard — to the outside world. ISDN2 carries two calls at once and is common in small offices. ISDN30 comes in blocks of up to 30 channels, one call per channel, and sits behind larger offices, hospitals, councils and contact centres. Both usually come with a block of direct-dial numbers.
A useful rule of thumb: if a site has more than a handful of desk phones on its own phone system and nobody has touched it in ten years, assume it's on ISDN until someone proves otherwise.
Where things stand
Openreach's latest estimate is around 1.2 million lines still on the old network, down from 1.5 million at the end of July. About 350,000 of the July figure were business premises — and the business number Openreach quoted in September hadn't moved since.
Nobody publishes a separate count of ISDN circuits. But the fact the business figure has stalled tells you something: the sites left aren't the ones who forgot. They're the ones where it's genuinely complicated.
What happens on the day
The line goes dead. There's no dial tone, and anyone ringing your numbers hears an unobtainable tone, so to a customer you look closed. Anything that dials out over the line — an alarm panel, a lift phone — fails silently. You find out when it's needed.
Your numbers need moving to a new provider while the old line is still live. Once a line is ceased, getting its numbers back is difficult and not guaranteed. Openreach has announced a last-resort safety net for some lines that miss the deadline, but it's temporary, costs more, and isn't something any business should plan around.
What else goes with it
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emergency
999 calls
If the desk phones are the only phones on site, nobody can call 999 from them. After you move, the address sent to emergency services has to be set correctly for every site.
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elevator
Lift emergency phones
Often plugged into a spare port on the phone system, so they get missed. They must work in a power cut, and a lift without one usually has to come out of service.
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notifications_active
Fire and intruder alarms
The alarm may still sound on site, but the monitoring centre never hears about it. Worth checking with your insurer.
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point_of_sale
Card machines, door entry, fax and franking
Older kit that dials out, still more common than people expect.
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headset_mic
Contact centres
Queues, menus, wallboards and call recording all sit behind the ISDN channels. Some older recorders tap the line directly and stop without anyone noticing. In a regulated business, that's a compliance problem, not an IT one.
What it's costing you now
Typical yearly line rental for one 30-channel ISDN30 circuit, before call charges, at £25–£50 per channel per month.
The replacement lines usually cost a fraction of that. Meanwhile, the analogue lines that often sit alongside ISDN for lifts and alarms are getting dearer fast: Openreach raised their wholesale price by 20% in April and 40% in July, and a further 40% rise on 1 October takes them to double their 2025 level. Add the cost of keeping an out-of-support phone system alive, plus the premium for anything done in a December rush, and waiting is the expensive option.
Your three options
| Route | Fits when | Main risk |
|---|---|---|
| Keep the phone system, swap ISDN for SIP trunks | The system is still supported and can take SIP (phone lines delivered over a data connection) | Relying on a single internet connection |
| Replace the phone system | It's out of support, or staff work from home some of the time | Call routing and queues don't transfer automatically |
| Drop the phone system, use mobiles | A small, mobile team with good signal everywhere | Losing shared lines, queues and in-building coverage |
My view: if the phone system is still supported, move the lines to SIP first and replace the system later. It turns one risky project with a hard deadline into two calm ones. Mobiles-only works for some businesses, but it's not as simple as it sounds — I've written about why.
Two things to get right whichever route you take. First, 30 calls need only around 3 Mbps, so capacity is rarely the issue — having a second connection by a different route is. Second, if the old system can't take SIP at all, a gateway box can buy time, but agree an exit date before you install it.
A quick word on VoIP phones
VoIP phones plug into the network socket rather than the phone socket. If you keep your phone system, your handsets stay. If you replace it, the digital handsets from older Avaya, Mitel or Panasonic systems usually won't come with you. Most VoIP phones are powered through the network cable, so they go off in a power cut unless your network kit has battery backup. And order early: popular models will go on backorder as January gets closer.
Common questions
Can we keep our phone numbers? expand_more
Yes, including your full block of direct-dial numbers, as long as they're moved to the new provider before the ISDN line is ceased. Never cancel the old line until the transfer has completed and been tested.
Do we have to replace our phone system? expand_more
Not if it's still supported and can take SIP trunks. If it's out of support, the switch-off is usually the right moment to replace it rather than pay to keep it limping on.
How do we know if we still have ISDN? expand_more
Check the phone bill for ISDN2, ISDN30, channels or a range of direct-dial numbers, and look near the phone system for a BT or Openreach box labelled ISDN.
Figures from Openreach's published updates (July and September 2026) and its legacy line price schedule (February 2026). ISDN rental range is typical of UK provider pricing.