ISDN switch-off: UK business guide to January 2027

The UK ISDN switch-off is part of the wider retirement of the Public Switched Telephone Network (PSTN). At present, BT plans to retire its PSTN by 31 January 2027, so businesses still using legacy phone lines should audit and migrate their services now.

In addition, the change affects more than voice calls. Alarm systems, lift phones, door-entry equipment, payment terminals, fax machines and other devices may also rely on an analogue or ISDN connection.

Specifically, this guide explains what is changing, what the deadline means and how to move to a suitable digital service without overlooking critical equipment.

When will ISDN lines terminate in the UK?

BT plans to retire its PSTN by 31 January 2027. Meanwhile, other communications providers that use BT’s network must work to the same timetable, while operators with their own networks may follow a similar but separate plan.

For simplicity, the deadline is often called the ISDN switch-off, PSTN switch-off, landline switch-off or digital phone switchover. However, these phrases can oversimplify the programme. The UK Government notes that most customers are expected to have migrated by the end of January 2027, but not every analogue phone service will necessarily be switched off by that date.

Therefore, businesses should confirm their own migration timetable with the company that bills and supports their service. Current information is available from Ofcom’s digital landline guidance and the UK Government’s analogue-to-digital transition guidance.

What is ISDN?

In short, ISDN stands for Integrated Services Digital Network. It is a set of standards that carries voice and data through the traditional circuit-switched telephone network.

For example, UK businesses commonly used ISDN2 and ISDN30 services to connect a private branch exchange, or PBX, to the public telephone network. ISDN2 usually served smaller requirements, while ISDN30 supported a larger number of simultaneous calls.

However, the technology was a significant improvement when introduced, but the underlying network is ageing and increasingly difficult to maintain. Modern services carry voice as data over IP networks instead.

What is the PSTN?

By comparison, the Public Switched Telephone Network is the traditional fixed telephone network used to connect landline calls. As a result, ISDN services operate within that wider environment, which is why the terms PSTN retirement and ISDN switch-off are often discussed together.

Ultimately, BT Group decided to retire the PSTN because the network is beyond its intended life and is becoming less reliable and more expensive to maintain. Ofcom supports the move to modern digital networks while monitoring how providers protect customers during migration.

In practice, Openreach manages much of the UK access network, but it is not the only company involved. A business normally buys its service from a communications provider, which is responsible for explaining the change and arranging its migration.

What will replace ISDN?

Generally, most business voice services will move to Voice over Internet Protocol, or VoIP. This sends calls as data over an IP connection rather than a dedicated ISDN circuit.

There are two common approaches:

• Hosted VoIP: the main phone-system functions run in a provider’s cloud platform. Users connect through IP desk phones, computers or mobile apps.
• SIP trunking: a compatible on-site or private phone system connects to the public telephone network over IP.

Therefore, the right option depends on the organisation’s existing equipment, locations, technical resources and appetite for change. Wayv’s business VoIP information gives an overview of the hosted approach.

Has the sale of new ISDN services already stopped?

Notably, Openreach introduced a national stop-sell for legacy Wholesale Line Rental products in September 2023. In practical terms, providers generally cannot order many new legacy lines or make certain changes once an area or product is under stop-sell rules.

However, a stop-sell is not the same as the final switch-off. Existing services may continue for a period, but that does not make delay risk-free. Hardware failure, contract expiry, office moves and service changes can become harder to manage as retirement approaches.

Consequently, a business should not base its plan on whether the old line still works today. It should agree a tested migration before the legacy service becomes a constraint.

Which business services could be affected?

First, start with every telephone number and line on the bill, then look for equipment that may not appear in the phone-system inventory. For instance, common examples include:

• ISDN2 and ISDN30 voice services
• analogue telephone and fax lines
• alarm signalling and monitoring systems
• lift emergency phones
• door-entry and intercom systems
• telecare and social-alarm devices
• payment terminals and franking machines
• building-management and telemetry equipment
• emergency or out-of-hours phones
• modems and remote-access devices

Above all, do not assume that an adaptor will make every analogue device reliable on a digital line. Ask the device manufacturer, maintainer and communications provider to confirm compatibility and testing requirements.

Why should businesses act before the deadline?

Ideally, a planned migration gives the organisation time to understand its dependencies, choose a suitable service and test the result. Waiting until the last minute increases the chance of rushed decisions, limited appointment availability or an unnoticed device stopping.

Furthermore, early work is especially important for several sites, regulated operations, care settings and any organisation with safety-critical equipment. It also provides time to coordinate number porting, staff training and contract dates.

In other words, the aim is not simply to replace one telephone product with another. Instead, use the project to check how customers contact the business, how calls are routed and what must keep working during an outage.

How to prepare for the ISDN switch-off

1. Build a complete line and number inventory

First, collect bills, contracts and number lists from every site. Then, record the provider, service type, location, contract end date and business owner. Include direct-dial numbers, published numbers and lines used only for equipment.

Then compare the paperwork with a physical site check. Labels are useful, but trace connections where possible and speak to facilities, IT, finance, security and operational teams.

2. Identify every connected device

Next, for each line, record what connects to it and why. Ask whether the device needs to dial out, receive calls, transmit data or remain available during a power cut.

Afterwards, contact the device supplier for written guidance on digital compatibility. A successful test call does not always prove that monitoring, fault reporting or emergency functions will work reliably.

3. Separate voice requirements from specialist services

For example, a modern business phone platform may replace the office voice system, but it may not replace a lift phone or alarm circuit. Treat each use case separately and involve the relevant specialist.

Consequently, this separation helps avoid a common mistake: porting the visible telephone numbers and cancelling the underlying service before hidden devices have been migrated.

4. Assess connectivity and local networks

In most cases, VoIP needs a stable IP connection. Review available bandwidth, latency, packet loss, Wi-Fi coverage and the number of simultaneous calls. Also check whether routers and switches can prioritise voice traffic.

Therefore, some locations may need a broadband upgrade or a separate connection. In addition, identify what will happen if the main circuit fails.

5. Design power and service resilience

By contrast, unlike some traditional corded telephones, digital phone equipment generally needs local mains power. A power cut can therefore stop the router, network switch, desk phone or local adaptor.

First, decide which calls and devices are critical, how long they need to work and what backup is proportionate. For example, options may include an uninterruptible power supply, a secondary circuit, mobile failover or call diversion.

Finally, test the complete chain. Backup power for one handset is not enough if the router or network equipment has already stopped.

6. Choose a suitable migration route

Next, compare hosted VoIP, SIP trunking and any specialist replacement service against the same requirements. Consider user experience, support, security, emergency calling, integrations and the total contract cost.

Wayv has information on business connectivity and communications, while this earlier guide summarises VoIP advantages. Keep the decision grounded in the organisation’s actual needs rather than a generic feature list.

7. Plan number porting carefully

In simple terms, number porting transfers a telephone number between providers. Give the new provider accurate account details and a complete list of associated numbers and services.

Do not cancel the legacy service before the port is confirmed. Doing so can delay the transfer or put numbers at risk. Agree the order of work, fallback plan and the person authorised to approve changes.

8. Pilot, test and train

Where possible, first start with a small group or less complex site. Test inbound and outbound calling, caller ID, transfers, voicemail, opening hours, call groups and emergency calling.

Then test specialist devices with their monitoring centre or maintainer. Record the result and any remedial work.

Finally, give employees short training based on their role. People who answer calls need different guidance from administrators who manage users and routing.

9. Communicate the change

Before launch, tell employees when the change will happen, what will feel different and where to get help. If customers need to use a new number or contact route, update websites, directories, signage and documents in a controlled sequence.

Hybrid working remains part of many organisations’ operating model; this historic workplace commentary is retained from the original guide. However, design decisions should be based on the current needs of your own team.

10. Decommission only after verification

Finally, keep a signed checklist of numbers, devices and tests. Once every service owner confirms the replacement works, arrange cancellation of the old product in line with the agreed plan.

Afterwards, monitor bills for missed lines and check that call routing, recordings and reports match the new design.

What should a business ask a VoIP provider?

• How will you assess our network and simultaneous call demand?
• What support hours, response targets and escalation routes apply?
• Who owns and controls our telephone numbers?
• How will number porting be planned and protected?
• What happens during a broadband or power failure?
• How are emergency calls and user locations managed?
• Which security controls and administrator permissions are available?
• Which integrations are supported and how is data handled?
• What training is included for users and administrators?
• What are the full charges, contract term and exit arrangements?

Above all, ask for clear answers in writing. A proposal should identify assumptions, exclusions and responsibilities rather than relying on vague promises.

ISDN switch-off checklist for businesses

• List every site, line, number and contract.
• Identify alarms, lifts, entry systems, payment devices, fax machines and other connected equipment.
• Confirm compatibility with manufacturers and maintainers.
• Assess broadband, local networks and call capacity.
• Design power, connectivity and call-routing resilience.
• Choose the right replacement for each service.
• Plan number porting without cancelling early.
• Pilot and test voice calls and specialist devices.
• Train users and administrators.
• Update contact information where required.
• Decommission legacy services only after sign-off.
• Keep evidence of decisions and test results.

ISDN switch-off FAQs

Is the ISDN switch-off still happening in 2025?

The timetable has changed. BT now plans to retire its PSTN by 31 January 2027. Businesses should use current provider, Ofcom and UK Government information rather than older 2025 deadlines.

Will every analogue service stop on 31 January 2027?

Not necessarily. The UK Government says most customers are expected to migrate by the end of January 2027, but not all analogue phone services will be switched off by then. Confirm the plan for each service with its provider.

Is ISDN the same as PSTN?

No. PSTN describes the traditional public telephone network, while ISDN is a digital service operating within that circuit-switched environment. Their retirement programmes are closely linked.

Can a business keep its existing numbers?

Usually, eligible numbers can be ported to a new provider. Accurate records and careful sequencing are essential, so never cancel the old service before receiving porting instructions.

Will the existing telephone handset still work?

It depends on the service and equipment. Some handsets may work through a router or adaptor, while others need replacement. Ask the provider to confirm compatibility and functionality.

Will digital phones work in a power cut?

Only if the equipment and connectivity have suitable backup. Routers, switches, handsets and local adaptors normally require power, so resilience must cover the whole service.

Does every business need hosted VoIP?

No. Hosted VoIP suits many organisations, but SIP trunking or a specialist device service may be more appropriate in some situations. Requirements should drive the choice.

What is the first action a business should take?

Create a complete inventory of lines, telephone numbers and connected devices. That audit reveals the scope of the migration and the teams or suppliers that need to be involved.

Plan the move before the deadline creates pressure

Ultimately, the ISDN switch-off is a business continuity project, not merely a telephone upgrade. A thorough audit, suitable connectivity, tested resilience and careful number porting will reduce disruption and protect overlooked services.

If you need help reviewing your current setup and migration options, contact Wayv before changing or cancelling any existing line.