ISDN switch-off 2027: migrate your business phone lines and find business fibre

Business guide, reviewed 21 June 2026

The quick answer

ISDN2 and ISDN30, the digital phone circuits most UK businesses use for their phone systems, are switched off with the rest of the copper network on 31 January 2027. If your business still runs on ISDN, you need to move to SIP trunking or a cloud phone system, and both run over the internet. So the sensible first step is to check what business fibre is available at your postcode.

Checks full fibre, SOGEA and business options at your address. Independent results, no sign-up.

What is the ISDN switch-off, and when does it happen?

ISDN is retired with the rest of the PSTN on 31 January 2027. The deadline is locked, new ISDN lines stopped being sold in September 2023, and more than 500,000 business lines still needed migrating as the final phase began.

ISDN (Integrated Services Digital Network) is the technology that has carried business voice over digital channels for decades, usually as ISDN2 for smaller sites or ISDN30 for larger phone systems. It runs on the same copper infrastructure as the analogue phone network, and BT Group is retiring all of it on 31 January 2027 (Openreach, 2026). Openreach stopped selling new ISDN and analogue lines on 5 September 2023, so you cannot add new circuits, only run down the ones you have (Openreach, 2025).

This is not a far-off problem. Around 2.8 million lines were still on the old network as the final phase began, more than 500,000 of them serving business premises, and the required migration rate is roughly 47,000 lines a week (Openreach, 2025, 2026). Sites that do nothing risk a forced disconnection on their exchange's switch-off date, which is more expensive and disruptive than a planned move.

What should my business replace ISDN with?

The three mainstream replacements are SIP trunking, which keeps a SIP-ready on-site PBX, a hosted cloud phone system, or Microsoft Teams calling. All three run over an IP connection, so business fibre or SOGEA needs to be in place first.

The three ways UK businesses replace ISDN in 2026, and who each suits.
ReplacementBest forKeeps your PBX?What you need
SIP trunkingBusinesses with a capable on-site phone system they want to keepYes, if SIP-readyFibre or SOGEA, plus a SIP-compatible PBX or gateway
Hosted cloud phone systemMost small and medium businesses wanting low maintenanceNo, it replaces itFibre or SOGEA, IP handsets or a softphone app
Microsoft Teams callingTeams-first businesses wanting calls in the same appNo, it replaces itFibre or SOGEA, Microsoft licences, a calling plan or Direct Routing

SIP trunking is the closest like-for-like swap if your phone system already supports it: you keep the handsets and call flows your team knows, and the SIP service simply replaces the ISDN circuits. A hosted cloud system or Teams calling removes the on-site hardware entirely, which suits businesses that would rather not maintain a PBX. Whichever route you pick, the underlying requirement is the same: a reliable internet connection good enough for clear calls.

Why do I need to check business fibre first?

SIP and cloud calling depend on a reliable internet connection for call quality. Checking what fibre is available at your postcode tells you whether you can run full fibre (FTTP) or SOGEA, and which uptime guarantee makes sense.

Every ISDN replacement carries your calls over the internet, so the connection becomes business-critical: if it drops, so do your phones. That is why the first practical step is not choosing a phone system, it is finding out what connectivity you can actually get at your premises. Full fibre (FTTP) is the strongest foundation, and where it is not built yet, SOGEA gives you a digital line without the old copper phone service. SOGEA reaches about 98.5 per cent of premises, and full fibre now reaches 82 per cent of UK homes with gigabit-capable coverage at 89 per cent (Openreach, 2025; Ofcom, 2026).

Start with your postcode

See the full fibre, SOGEA and business options available at your address, ranked by what they actually cost over the term.

Prefer to browse first? See the business broadband hub and how business broadband and leased lines compare.

For always-on sites such as shops, surgeries and contact centres, ask the provider about a service-level agreement with an uptime commitment and a target fix time, and consider a 4G or 5G backup so a single fault does not take your phones offline. Our guide to business broadband with 4G backup covers that resilience layer.

How much does staying on ISDN cost in 2026?

More each quarter. Openreach is doubling the wholesale price of its copper line products in 2026, with rises on 1 April, 1 July and 1 October, so legacy line and ISDN costs climb steadily before the network even closes.

To push the final migration, Openreach is raising the wholesale price of its Wholesale Line Rental (WLR) copper products by 20 per cent on 1 April, by a further 40 per cent of the base price on 1 July, and by another 40 per cent on 1 October 2026, which doubles the line over the year (Openreach, 2025; ThinkBroadband, 2025). For a business running several legacy circuits, that adds up quickly, and in many cases a digital service is already cheaper than the post-rise copper price. The full pounds-and-pence schedule is on our copper line price rise tracker.

How do I migrate without losing my numbers or having downtime?

Plan a parallel cutover. Order the new connection, port your numbers (typically 10 to 15 working days), keep ISDN live until the new service is tested, then cease the old circuits.

  1. Check business fibre at your postcode. Confirm whether full fibre or SOGEA is available, because it shapes the phone solution and the timeline.
  2. Audit channels and numbers. Record how many ISDN channels you use, and list every number, including direct dial-in (DDI) ranges and any fax or published numbers, so all of them can be ported.
  3. Choose SIP or cloud. SIP trunking keeps a SIP-ready PBX; a hosted system or Teams replaces it. Look for a clear uptime service-level agreement and UK support.
  4. Order and run in parallel. Provision the fibre or SOGEA line and keep ISDN active until the new system is tested under real call load.
  5. Port and switch over. Port the numbers, test inbound and outbound calls, then cease the ISDN circuits.

Number porting is usually straightforward but takes time, so start early: missing even one published number can disrupt customers. Our guide to switching business broadband without downtime covers the cutover in more detail, and if you rely on a fixed public address, see the static IP business broadband guide.

What about my alarms, lifts and card machines?

ISDN is only the voice side. Alarms, lift emergency phones, card machines, fax and door entry that use copper also stop working at the switch-off, and each needs its own migration path.

The switch-off affects far more than your phone system. Intruder and fire alarms, lift emergency phones, card terminals that dial out, fax and CCTV diallers all need moving to IP or cellular alternatives, and some are safety-critical or carry insurance implications. Our SME PSTN business continuity hub is the full device-by-device checklist, and we have dedicated guides for card machines and EPOS and for care alarms and monitored alarms.

Questions people ask

Is the 2027 deadline really fixed?

Yes. ISDN and the wider PSTN close on 31 January 2027, the date is locked, and the stop-sell on new copper and ISDN lines has been in force since September 2023 (Openreach, 2025, 2026). Lines not migrated by the deadline move to a basic emergency voice service until they are switched across.

What is the difference between SIP trunking and a cloud phone system?

SIP trunking delivers calls to an on-site PBX over the internet, so you keep your existing phone system if it is SIP-ready. A cloud (hosted) phone system removes the on-site hardware and runs everything from the provider's platform, accessed through IP handsets or an app.

Do I need full fibre, or is SOGEA enough?

Either can carry SIP or cloud calls. Full fibre (FTTP) is the strongest and most future-proof option; SOGEA is a good interim where full fibre is not built yet, reaching about 98.5 per cent of premises (Openreach, 2025). A postcode check shows which you can get.

How long does the migration take?

It depends on the connection lead time and the size of your estate, but number porting alone typically takes 10 to 15 working days, so plan several weeks and run ISDN in parallel until the new system is tested.

Will my business broadband qualify for One Touch Switch?

A gaining-provider-led switching process for business is being introduced during 2026, and business lines already follow most of the switching rules. Your new provider increasingly manages the process, but confirm the exact steps with them.

Further reading and official guidance

We link to the official guidance and to competitor comparisons so you can benchmark business options widely.

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