The UK's traditional analogue phone network, the PSTN, is being switched off. Openreach says that by 31 January 2027 all traditional phone lines will be going digital, and BT Business says ISDN services will stop working from that date unless they have been moved to an IP-based service. Any business still making calls over analogue or ISDN lines, or running equipment that depends on them, needs to move before then.
When is the PSTN switch-off?
Openreach, which runs most of the UK's phone network, has set 31 January 2027 as the date by which all traditional phone lines will be going digital. In its update of 31 July 2026, Openreach estimated that around 1.5 million lines were still on the old copper network, including around 350,000 business premises.
What actually stops working?
Two kinds of service depend on the old network:
- Analogue phone lines. Openreach says they are being replaced by digital phone lines, which usually work through a broadband router rather than a wall socket.
- ISDN lines (ISDN2 and ISDN30), the multi-channel lines many offices and contact centres use to connect a phone system. BT Business says these cannot be moved automatically and will stop working from 31 January 2027 if they have not been migrated.
What should businesses do now?
- List every line and number. Your phone bills show which lines you pay for and what each number is used for.
- List every connected device. Walk the building: alarm panels, lift phones, door entry, card terminals, franking machines and fax lines.
- Ask each device supplier whether the equipment works on a digital line, and what it needs if not.
- Choose the replacement for voice. A phone system or contact centre platform usually moves to SIP trunking; smaller sites often move to a cloud phone service such as UCaaS.
- Keep your numbers. Arrange number porting as part of the move, so customers keep calling the numbers they know.
- Plan the cutover and test it, including what happens to calls in a power cut, because a digital line depends on powered equipment such as the router.
Can we keep our phone numbers?
Yes. Moving to digital voice changes how calls are carried, not the number itself. Existing geographic and non-geographic numbers can be ported to the new service. DigiTalker's number management page sets out how porting works and how long it usually takes.
Why SIP trunking replaces ISDN
A SIP trunk connects a phone system (an on-premise PBX or a cloud contact-centre platform) to the public telephone network over an internet or private data connection instead of ISDN lines. Capacity is counted in channels, one per simultaneous call, and can be changed in software rather than by installing new lines. Because voice travels as data, call quality depends on enough bandwidth and properly configured quality of service on the network.
| Feature | ISDN | SIP trunking |
|---|---|---|
| Status after 31 January 2027 | Withdrawn | Unaffected: runs over IP |
| How capacity is added | New physical lines | More channels, set in software |
| Existing numbers | Must move before the switch-off | Ported onto the trunk |
| What it depends on | The old copper network | A data connection with enough bandwidth |
How DigiTalker can help
DigiTalker provisions SIP trunks, ports existing numbers and connects them to your phone system or contact centre platform. If you are still on analogue or ISDN lines, the sooner the migration is planned, the more time there is to test before the deadline.
Frequently asked questions
When is the PSTN switch-off?
Openreach says that by 31 January 2027 all traditional phone lines will be going digital. BT Business says ISDN2 and ISDN30 services will stop working from the same date if they have not been moved to an IP-based service.
Can we keep our existing business phone numbers?
Yes. Moving to digital voice changes the line, not the number. Existing geographic and non-geographic numbers can be ported to the new service so callers keep using the numbers they already know.
Which devices should we check before the switch-off?
Anything that uses a phone line rather than a data connection. Openreach lists alarms, CCTV, payment terminals, telecare and health monitoring devices, door entry systems and lift phones. Ask each supplier whether the device works on a digital line.