BT PSTN & ISDN switch-off 2027: how it really affects your business

What's happening — and why

BT and Openreach are switching off the entire UK public switched telephone network — known as the PSTN — and with it the ISDN lines that sit on top of it. The original switch-off date was December 2025. In late 2023 it was postponed to 31 January 2027 to give vulnerable customers, telecare users and critical-service providers more time to migrate safely.

Stop-sell already happened. No UK carrier can take a new PSTN or ISDN order anywhere in the country. If your existing lines come up for renewal, you can't simply re-sign — the only path forward is an IP replacement.

For most UK businesses still running ISDN this is the biggest forced telecom change since copper landlines arrived. It isn't optional, it isn't a sales pitch, and the timeline isn't flexible. The good news: done properly, most businesses end up with a better phone system and a smaller monthly bill.

PSTN vs ISDN — what's the difference?

PSTN (Public Switched Telephone Network) is the underlying analogue/digital voice network that BT and Openreach have run since the early 20th century. Every traditional UK landline phone — the one plugged into a copper socket on the wall — runs over PSTN.

ISDN (Integrated Services Digital Network) is a business-grade service that BT launched in 1986 to give companies multiple digital voice channels over a single line. Two flavours are common in UK businesses:

• ISDN2 — 2 simultaneous calls per line. Common in small offices and as a backup for analogue.
• ISDN30 — 30 channels, often sold in 8/15/20/30 packs. The standard call-centre / mid-large business setup for the last 30 years.

Both ISDN and PSTN are being switched off in the same event. Different products, same underlying retiring network.

The real reason behind the switch-off

It's a commercial decision driven by infrastructure cost. The kit that runs PSTN — physical telephone exchanges with decades-old switching gear — is increasingly expensive to maintain, hard to source parts for, and consumes a lot of power and floor space. Openreach has already moved its fixed broadband estate onto IP (fibre-to-the-cabinet and fibre-to-the-premises), and voice was always going to follow.

Once all voice runs over IP, the old analogue switches can be retired and the exchange floors repurposed. That's the entire business case.

What replaces ISDN and PSTN?

Voice over IP. Specifically:

• SIP trunking — for businesses that already have their own PBX (3CX, Asterisk, FreePBX, Cisco, Mitel, Avaya, Panasonic, etc.). The trunk replaces the ISDN circuit, runs over your broadband or leased line, and your existing PBX keeps handling the call logic. Plain-English SIP trunking guide →
• Hosted cloud PBX— for businesses that don't want to run their own phone system anymore. The PBX itself lives in the cloud, your desk phones and softphones register to it over the internet, and you stop having a box in the comms cupboard. Smart features (IVR menus, hunt groups, time-of-day routing, voicemail-to-email, call recording) become free instead of bolt-ons.
• Softphones and mobile apps— for staff who work from anywhere or don't need a desk phone at all. A laptop or mobile registers to the same hosted PBX and behaves identically to a desk extension.

BT's consumer replacement is branded Digital Voice: same idea, sold to households over a fibre broadband line. Businesses are not the target of Digital Voice; the business path is SIP trunking or hosted PBX, usually bought from a non-BT provider.

What your business needs to do before 31 January 2027

Treat this as a project, not a panic. There are five steps:

• 1. Inventory what's on PSTN/ISDN today. Voice lines obviously, but also anything else that quietly dials out: fax, alarm panels (intruder and fire), lift telephones, door-entry intercoms, EPOS card terminals, franking machines, building-management systems, lone-worker devices, and emergency lift autodiallers. The hidden non-voice equipment is what catches most businesses out.
• 2. Decide your replacement model. Keep your existing PBX (SIP trunks) or move the whole system to the cloud (hosted PBX). The honest deciding factor is the age and capability of your existing PBX. If it's SIP-capable and less than ~10 years old, SIP trunks are usually cheaper. If it's older, replacing the PBX with a hosted one is usually cleaner.
• 3. Check your internet connection. VoIP needs reasonable bandwidth and low jitter. Rule of thumb: ~100 kbps up and down per concurrent call (so a 10-person office needing 10 simultaneous calls wants ~1 Mbps dedicated to voice on top of everything else). A standard FTTC line is fine for most SMEs; FTTP is better if it's available. If you're on slow ADSL, plan the broadband upgrade before the voice migration.
• 4. Plan the number porting. All UK numbers are portable by law (General Conditions C7). You keep your 0800s, your geographic numbers, your hunt groups — everything ports across. With us, port-in is free with Membership. Plan for 5–10 working days for a typical port, longer for complex ISDN30 estates.
• 5. Cut over with a defined window. Don't flip everything at once. Port one or two numbers first to test routing and call quality, then migrate the rest in phases.

The closer you leave it to January 2027, the longer port queues get and the less leverage you have on your supplier. Realistic planning window: start the project at least 6 months before you want to be live.

The practicalities of switching from ISDN

Switching from ISDN to VoIP isn't as complicated as the scale of the change makes it sound. The bones of the project:

Is your internet connection good enough?

VoIP itself is light on bandwidth (~100 kbps per concurrent call), but it's sensitive to jitter and packet loss. Prioritise voice traffic on your router (QoS) and SIP holds up fine on even modest connections. A small office (5–10 simultaneous calls) is comfortable on a standard 80/20 FTTC line. Larger offices, or anywhere you can get it, FTTP is the upgrade. We don't sell broadband, but we'll tell you honestly whether your current circuit will hack it.

Does your office phone system support VoIP?

Modern PBXs (3CX, FreePBX, Asterisk, Cisco, Mitel, Avaya) all support SIP natively. Older traditional PBXs sometimes need an SBC (session border controller) or ATA (analogue telephone adapter) to bridge the protocol gap — which is possible but rarely worth it for a system already past its useful life. If your PBX is more than 10 years old and your handsets are tired, this switch-off is the moment to replace, not bridge.

Cloud PBX vs keeping the box on-prem

Replacing a physical PBX with a hosted cloud one gives you most of the upside: nothing on-site to maintain, instant scaling, smart features included, web-based admin panel. With Membership you don't pay separately for the PBX licence — it's part of the £149/mo flat fee.

What about alarms, lifts, door entry, EPOS?

This is the bit a lot of switch-off articles skip. PSTN carried plenty of non-voice traffic on the side: anything that quietly dials a number to report back to a monitoring centre. After January 2027 those copper lines stop working.

Common "hidden" PSTN devices that need attention:

• Fire alarm autodiallers — replace with an IP-based or 4G GSM dialler. Confirm your fire alarm contractor has factored this in.
• Intruder alarm signalling — most monitored intruder alarms still dial out over PSTN. Upgrade to a dual-path (IP + GSM) signalling unit.
• Lift autodiallers (BS EN 81-28) — every passenger lift in the UK must have a working two-way emergency voice line. PSTN-based lift phones must be migrated to GSM or VoIP.
• Door entry / intercom systems — older systems that dial out to a flat or office over PSTN need replacing.
• EPOS card terminals — most modern terminals are already IP or 4G, but legacy dial-up terminals will stop processing payments.
• Franking machines — Royal Mail-approved terminals that dial out to reload credit. Most have IP/4G alternatives.
• Building-management and HVAC — some BMS systems still use PSTN dial-out for fault reporting.
• Lone-worker devices — anything in the field that calls a monitoring centre.

None of these are NumberGroup's domain directly, but we mention them because they catch businesses out. Add them to your switch-off audit.

What the replacement actually costs

The headline number everyone wants: £149/month flat with NumberGroup Membership covers everything most SMEs need to replace ISDN entirely.

• SIP channels — Free, up to 25 concurrent calls included.
• UK numbers — Unlimited geographic, 0800, 0808, 03. Free.
• Port-in of all your existing numbers from BT or any other provider — Free.
• Hosted PBX with smart features — IVR, hunt groups, time-of-day routing, voicemail-to-email, call recording. All-in.
• UK call rates — passed through at wholesale: roughly 0.3p/min to UK landlines, 0.6p/min to UK mobiles. See the full call rate card.
• Hardware — Grandstream IP desk phones at our distributor cost (GXP1625 £28, GXP2135 £64, GXP2170 £83 ex-VAT). No handset markup.
• UK support team — a small team that knows your business, not a ticket queue. Answers in seconds during UK business hours.

Need more than 25 concurrent calls (large call centre, multi-site, high-volume sales floor)? Tell us about your peak and we'll quote based on real volume — no per-channel licence fees, no minimum commit.

The honest typical saving— most ISDN30 businesses are paying £150–£400/month in line rental alone before they make a single call. Moving to Membership at £149/mo means most save the fee in their first bill, every month afterwards is upside, and the bill never includes "line rental" as a line item again.

Will I keep my phone numbers?

Yes. UK number portability is a legal right under Ofcom's General Conditions C7. Every UK telephone number is portable to another licensed UK carrier, and the receiving carrier (us) does the work — you fill in a porting form, we lodge it with the losing provider, and the cut-over happens within a defined window agreed in advance.

With Membership, port-in is free. Caller experience is identical before and after — same number, same ring, no announcement.

One thing to know: if your numbers sit on an ISDN30 with a sprawling DDI block (say 100 consecutive numbers), the port is technically straightforward but takes longer to coordinate. Start that conversation early.

Getting help

We're helping UK businesses move off ISDN every week. Number porting, hardware setup, PBX migration, the non-voice stuff (alarms, lifts) we'll flag even though we don't fix it — it's all done by our UK onboarding team and we don't charge extra for the handholding.

If your ISDN renewal is on the horizon, or you just want an honest second opinion on a quote you've been given, get in touch or ring 0800 310 1010. No obligation, no sales call, no follow-up sequence.