The BT VoIP Phone Switch | What UK Businesses Need to Know

What is the BT VoIP switch?

The BT VoIP switch is BT and Openreach decommissioning the Public Switched Telephone Network — the copper-wire network that has carried UK phone calls for over a century — and replacing it with fibre and IP infrastructure. Once your exchange is switched, copper-based voice stops working and every service must run over VoIP.

In practice, anything tied to the old copper network will eventually go dark: traditional analogue business lines, ISDN2, ISDN30, and ADSL broadband over copper. Voice has to move to Voice over IP, which sends calls as data packets across a broadband or business leased line connection rather than a dedicated copper pair.

This is not a BT marketing exercise. It is a national infrastructure programme that every UK business with a phone line has to plan for. Treating it as an IT project — not a telecoms afterthought — is how you avoid being caught short.

When is the PSTN switch-off happening?

The PSTN switch-off is running exchange by exchange and completes nationally by January 2027, the date Openreach has set for the final stop-sell and migration of all remaining lines (Openreach). Pilot programmes began in 2021, many exchanges are already fully migrated, and the remaining areas are being worked through during 2025 and 2026.

The shift is already well advanced. Legacy voice (PSTN) connections now account for only 19% of all UK landline connections — down from 27% in 2024 and 41% in 2023 — per Ofcom Connected Nations 2025. Over two-thirds (>67%) of UK landlines have already been upgraded to VoIP or digital voice (2025 UK data).

What has already changed across most exchange areas:

• ISDN2 and ISDN30 services have been ceased in the majority of UK exchanges
• New PSTN and ISDN line orders are no longer accepted in most areas
• Broadband over legacy copper is being phased out wherever full-fibre is live

BT and Openreach write to customers in areas approaching switch-off, but you should not wait for a letter. Businesses are repeatedly caught out by compressed timescales after assuming they had years left.

What does the VoIP switch mean for your business?

It means four practical changes: your calls travel over your internet connection, some hardware needs replacing, secondary lines (alarms, lifts, fax) need migrating, and call quality becomes a function of your broadband. Each one is manageable, but each one is a place migrations go wrong when nobody owns the detail.

Calls travel over your internet connection. Once on VoIP, every call is data over your broadband or leased line. Call quality now depends directly on connection quality and reliability. A stable, business-grade connection — ideally FTTP or a leased line — is strongly recommended for any business that treats the phone as a primary channel.

You may need new hardware. Desk phones built for PSTN or ISDN will not work with VoIP. You will need IP phones built for VoIP, softphone apps on laptops and mobiles, or analogue telephone adaptors (ATAs) if you want to keep specific existing handsets.

Alarm, lift and fax lines need special attention. Secondary lines for lift emergency phones, door entry, CCTV backhaul and fax are the single most overlooked part of any migration. They must all be migrated or replaced — fax usually moves to an internet fax service, while alarm and lift lines often need sign-off from the installer or manufacturer.

Call quality depends on your broadband. A single VoIP call uses roughly 80–100 kbps, so ten concurrent calls need about 1 Mbps — trivial for most connections. Problems start when voice competes with other traffic on an unmanaged line. Quality of Service (QoS) on the router, or a dedicated voice VLAN, keeps calls clean under load.

How do you prepare for the BT VoIP switch?

Preparation is a five-step process: audit your lines, review your connectivity, choose a provider on merit, plan number porting, then replace or adapt hardware. Do it in that order and the migration becomes a scheduled change rather than an emergency. Below is the sequence AMVIA runs for clients.

1. Audit your existing lines. Establish exactly what lines you have, what each is used for, and which systems depend on them. Include PSTN lines, ISDN channels, alarm lines, lift lines and fax. Most businesses uncover lines they had completely forgotten.

2. Review your broadband connectivity. VoIP needs a reliable, business-grade connection. A consumer-grade FTTP line can introduce quality issues for multiple concurrent callers. A dedicated leased line gives uncontended bandwidth and guaranteed latency — the right foundation when voice is mission-critical.

3. Choose a VoIP provider on merit. Do not default to whatever BT offers as your migration path. Independent and hosted platforms often deliver more features at lower cost. A hosted phone system or Microsoft Teams calling can replace an ageing PBX entirely — weigh providers such as Gamma Horizon, 3CX, RingCentral, 8x8 and Microsoft Teams Phone against your actual requirements.

4. Plan number portability. Your existing BT numbers can be ported to your new provider. Number porting typically takes 5–10 working days and is coordinated between the new provider and BT's wholesale porting team. Test and go live on the new system before the old lines are ceased.

5. Replace or adapt hardware. Arrange IP handsets, softphone licences or ATAs as needed. Fully remote or hybrid teams may only need a softphone app; businesses tied to physical desk phones can take a like-for-like IP replacement.

Is the switch-off a risk or an opportunity?

It is both — and the businesses that treat it as an opportunity come out ahead. The obligation is to move off copper. The opportunity is to retire an ageing on-premise PBX and gain call recording, analytics, mobile working and CRM integration that the old system never offered, usually at lower running cost.

Many UK businesses still have essential services — alarms, lifts, door entry and payment terminals — that depend on the PSTN and need to prepare for the 2027 deadline. Leaving it late forces a rushed, like-for-like swap. Planning early lets you redesign how your business communicates — and fold connectivity, voice and security into one accountable plan. That is AMVIA's model: one provider, security-first, Microsoft-certified.

AMVIA works with UK businesses to plan and run smooth PSTN migrations — handling the connectivity review, VoIP provider selection across the business VoIP market, number porting and hardware provisioning as a single managed process. If you also want to understand the wider deadline, our PSTN switch-off guide covers the national timeline in detail, and business broadband options sit alongside it.