The PSTN switch off is the retirement of the UK's analogue phone network, with all remaining PSTN and ISDN lines due to stop working by 31 January 2027. If your business still uses traditional landlines, ISDN channels or anything plugged into a copper line, you'll need to move to internet-based calling (VoIP) and check every connected device before the deadline.
What the PSTN Switch-Off Means for Your Business
Picture your card machine failing mid-sale, or your lift alarm going silent — all because the line behind them quietly stopped working. That's the practical risk of the PSTN switch off if you leave it until the last minute. The Public Switched Telephone Network (PSTN) is the analogue copper system that has carried UK voice calls for decades, alongside ISDN, the digital-over-copper service many businesses use for multiple lines.
Openreach is withdrawing both. From 2027 there will be no analogue or ISDN voice service — calls move to Voice over IP (VoIP), which carries them over your broadband or a dedicated internet connection. For most offices the desk phones and numbers can stay; what changes is the underlying line and the kit that depends on it.
The important detail competitors gloss over: this isn't only about phones. Anything that dials out over a copper line is affected — card payment terminals, door entry systems, lift emergency phones, fire and burglar alarm diallers, CCTV backhaul, fax machines and some older EPOS equipment. A phones-only migration that ignores these leaves dangerous gaps.
Key Dates and the 2027 Deadline
The headline date is 31 January 2027 — the point by which the PSTN and ISDN networks are switched off nationally. The original target was December 2025; it was extended to give businesses, vulnerable users and critical-service providers more time to migrate safely.
Two earlier milestones matter more than most businesses realise. A national Stop Sell is already in force: you can no longer buy new PSTN or ISDN lines, or make certain changes to existing ones, across the UK. In practice that means if you try to add lines to an old system, you'll be pushed onto a digital product anyway.
Openreach is also working through the copper network area by area, migrating exchanges ahead of the final date. Some London postcodes will be moved well before January 2027, so your effective deadline may be sooner than the national one. As a guide, treat the next 12 months as planning time, not spare time.
- Already live: Stop Sell — no new PSTN/ISDN lines or upgrades
- Ongoing to 2027: exchange-by-exchange migration, some London areas early
- 31 January 2027: full PSTN and ISDN switch-off — legacy lines stop working
Are They Really Getting Rid of Landlines?
Yes — but not phone numbers or the ability to make calls. The 'big switch off' ends the analogue delivery method, not landline calling itself. You keep your geographic numbers (your 020 London numbers port across), and you can keep physical desk phones if you want them. The call simply travels over an internet connection instead of a copper pair.
So 'landlines going digital' is the accurate way to think about it. A digital line behaves much like the old one day to day — you dial, it rings — but it's delivered as data. That unlocks features the copper network never could: call routing, voicemail-to-email, hunt groups, and the ability to take your number anywhere with an app.
The one genuine trade-off is power. Analogue phones drew power down the line and worked in a blackout; VoIP needs mains electricity for your router and handsets. That matters for critical-safety lines, which we cover below.
Moving to VoIP: Options and Costs
There's no single 'VoIP' — the right choice depends on how many users you have and how you work. The three common routes are a hosted VoIP system (SIP phones or softphones on a cloud platform), SIP trunks feeding an existing on-site phone system you want to keep, or Microsoft Teams as your phone platform if you're already in Microsoft 365.
As a rough guide for UK SMEs, a fully hosted cloud hosted PBX typically costs around £8–£15 per user per month, often including UK call bundles. SIP trunking to keep a capable existing PBX can work out cheaper per channel and suits businesses with recent hardware. If your team lives in Teams, a Teams phone system adds calling to the tools they already use and can reduce the number of separate apps.
Don't fixate on headline price alone. Weigh call quality, number porting, support response times and whether the provider will handle the migration end to end. The cheapest per-user tariff is a false economy if porting drags on or your 020 numbers go dark for a day.
- Hosted VoIP / cloud PBX: ~£8–£15 per user/month, lowest hardware burden
- SIP trunks: keep a modern on-site PBX, pay per channel
- Microsoft Teams calling: best if already standardised on Microsoft 365
Preparing Your Broadband and Network
VoIP is only as good as the connection underneath it. A handful of calls will run fine over decent FTTP broadband, but a busy office running VoIP alongside cloud backups, Microsoft 365 and video calls needs headroom and, ideally, resilience. Contention and upload speed matter more for voice than raw download figures.
For call-dependent businesses — think a London practice or contact team where dropped calls cost money — a leased line gives a dedicated, uncontended connection with symmetrical speeds and a firm service level. Where that's overkill, prioritising voice traffic on the network (QoS) and sizing your upload bandwidth sensibly goes a long way. Our network consultancy team can model expected call volumes against your connection.
Plan for the connection failing, too. Because voice now rides on broadband, a line fault takes your phones with it unless you've planned around it. A backup connectivity option — a second circuit or 4G/5G failover — keeps calls flowing. Pair that with UPS battery backup on your router and core switches so a brief power blip doesn't drop every call.
The Critical Lines Most Businesses Forget
This is where migrations go wrong. Beyond desk phones, copper lines quietly power safety-critical and revenue-critical systems. Lift emergency phones and auto-diallers, fire alarm monitoring, intruder alarm signalling, telecare, and PDQ card terminals can all stop working the moment their line is withdrawn — sometimes without obvious warning.
Treat these as a separate workstream. For each device, confirm with the equipment supplier or maintainer whether it's compatible with a digital line, needs an IP-based replacement, or should move to a mobile/roaming SIM. Lift and alarm lines in particular may need engineer visits and recommissioning, which take time to schedule.
If you're already modernising, it's a sensible moment to review connected building systems together — door entry, cloud CCTV and alarms — so they sit on properly specified, resilient infrastructure rather than being bolted onto consumer broadband as an afterthought.
A Simple Migration Checklist for SMEs
You don't need to boil the ocean. A structured audit-then-migrate approach removes most of the risk. Start now while there's slack in the timetable — rushing a port or an alarm reconfiguration against a hard deadline is where disruption happens.
Work through the list below in order. The first two steps alone will tell you how big your project really is.
- Audit every line and everything attached to it — phones, ISDN channels, card terminals, lifts, alarms, door entry, fax, CCTV
- Confirm your numbers and arrange porting so no 020 numbers are lost
- Choose your platform — hosted VoIP, SIP trunks or Teams calling
- Check broadband capacity, add QoS, and plan failover plus UPS backup
- Get supplier sign-off on every critical/safety line's digital compatibility
- Pilot with one team, then roll out and decommission old lines before the exchange migrates
How CLI Secure Helps London Businesses Migrate
We handle the whole switch as one project rather than leaving you to coordinate a telecoms provider, an ISP and an alarm company separately. That starts with a full audit of your lines and connected equipment, so nothing critical is missed — the step that catches out most DIY migrations.
From there we match the platform to how you actually work, port your numbers cleanly, and make sure the connection underneath is sized and resilient enough to carry voice reliably. For multi-site operators we can standardise across locations with multi-site connectivity and consistent call handling.
If you're weighing options or just want a sanity check on your timeline, get in touch — the earlier you start, the more choice and less pressure you'll have before 2027. You can reach our team via our contact page.