If you’ve taken a few telecoms calls in the past year, you’ve probably been told about three or four “deadlines”. The 2G shutdown. ISDN Stop Sell. Your current contract’s end date. Your provider’s “Q3 migration window”. They all sound similarly urgent. They aren’t.
There is one telecoms date in the UK calendar that is a hard network deadline. The kind that doesn’t move, doesn’t have exceptions, and doesn’t care about your buying preferences. That date is end of January 2027, when BT and Openreach finish switching off the PSTN and ISDN networks that have carried UK business voice for forty years.
This post is about why that date is structurally different from the others, what physically stops working on the day, and why “we’ve got eighteen months” is shorter than it sounds.
What actually stops working
PSTN is the Public Switched Telephone Network: the copper-and-exchange voice network UK businesses have been on since the 1960s. ISDN is the digital overlay on top of it — ISDN2e for two channels, ISDN30 for thirty. Openreach’s plan is straightforward: every copper-voice service that runs over PSTN or ISDN ends by end of January 2027.
The casualty list is broader than people expect.
- Every “BT landline” used as a business voice line.
- Every ISDN30 trunk feeding a traditional on-premise PBX.
- ISDN2e on small offices with under ten lines.
- “Featureline” and “Featurenet” services, which were already a managed product on top of PSTN.
- Anything else dialling out over a copper voice channel: alarm-monitoring dialers, lift emergency phones, EFTPOS terminals on a phone line, fax machines, building-management telemetry, GP-surgery emergency-button systems.
The phones themselves are the obvious part. The invisible casualties — the alarm dialer that’s not on your IT inventory, the lift phone the building maintenance company supplied a decade ago — are where most of the day-of surprises hide.
The other “deadlines” people quote you
Three other dates come up a lot in sales conversations. They are real, but they are not in the same category.
2G/3G sunset. EE shut down 3G in 2024; Vodafone, Three, and Virgin Media O2 are following on staggered schedules. This affects mobile devices and IoT modules using 2G or 3G fallback. It does not affect your office phone system unless you have a 2G-based GSM gateway as a backup line. Worth checking, but it’s not a structural threat to your phones.
ISDN Stop Sell (September 2023). Openreach stopped accepting new ISDN orders three years ago. You can’t add new lines, but the existing ones keep running until January 2027. Stop Sell isn’t a deadline you need to meet; it’s a deadline that already passed and only affects you if you’re trying to expand.
Contract end dates. A contract end is a renewal opportunity, not a network event. Letting your contract auto-renew on a phone line that’s about to be shut off anyway is a paperwork problem on your provider’s side, not a deadline on yours.
Local exchange “migration windows”. Some exchanges are closing earlier than the national deadline. If your address is in a London or Manchester migration tranche, your local deadline might be Q3 2026. We check this at audit; the public Openreach exchange list is available but hard to read at a glance.
Why eighteen months is short
It feels like plenty of time. It isn’t, for three structural reasons.
Engineering availability. Every UK business on PSTN or ISDN needs an installer. The pool of people who can pull a desk-phone install, configure a SIP gateway, and migrate a number is finite. Late-2026 calendars are already starting to fill up in busier postcodes — central London, Edinburgh, Manchester, Bristol. The closer to the deadline, the longer the lead time.
Number porting timelines. Porting a single number from BT to a new SIP provider takes 10–30 business days. Multi-number block ports take longer. If you have twelve numbers and you start the port in December 2026, you are already over the line.
Decision-to-deploy lag. From signing up to going live, a typical 35-user migration is 2–4 weeks. From “I should probably look at this” to signed contract, in our experience, is anywhere from 2 weeks to 6 months. The number you should be working backwards from is “fully migrated by October 2026”, which gives slack for a port hiccup, a delayed engineer visit, or a Christmas freeze.
The Q4 2026 window will be brutal for late starters. Auditing now costs nothing and locks your slot in the calendar.
What an audit catches
A PSTN-readiness audit takes 15 minutes and tells you four things.
- What you’re actually on. People often think they’re on a modern hosted system when they’re on a hybrid setup with ISDN30 trunks feeding a Mitel or Avaya PBX. The bill doesn’t always say.
- Which non-obvious devices need a plan. Alarm dialers, lift phones, payment terminals, fax machines. We walk through them with you.
- What your local exchange’s closure date is. Some are earlier than the national January 2027 deadline.
- What it costs and how long the migration takes. Fixed-price quote, broken out line by line. The cost stack is published, so you can compare ahead of the conversation.
Most audits end with “you’ve got time, here’s the order to do things in”. Some end with “you should start porting in the next 60 days”. Either way, you find out before the calendar starts working against you.
What we do
We run a managed 3CX service for UK SMEs facing the switch-off. We do the audit, supply the handsets, port the numbers, configure the system, train your team, and answer the phone when something needs fixing. One supplier across licence, hosting, SIP, handsets, and support.
You don’t have to use us. You do have to migrate. The full mechanics of how the migration works are on our PSTN switch-off guide. If you’d like a second opinion on what’s involved at your specific site, book a free audit. 15 minutes. No commitment.