Quick answer
The PSTN (Public Switched Telephone Network) is the UK's traditional analogue phone network - the copper lines and exchanges that have carried voice calls for over a century. ISDN is the digital business service built on the same network in the 1980s to carry multiple calls into an office phone system. Openreach, which runs the network, has stopped selling both and is switching them off, with the national deadline of 31 January 2027. Anything still plugged into a copper phone line then - phones, but also alarm signalling, lift emergency lines, older card terminals, fax machines and door entry - stops working. The replacement for voice is hosted telephony (or SIP trunking for businesses keeping a PBX), running over a broadband connection such as SoGEA or full fibre, and it is usually cheaper than what it replaces. The other devices each need their own digital replacement.
This guide is the technology explainer: what these networks are, why they are going and what replaces each part. If you want the deadline and the order of work, read the PSTN switch-off deadline guide; for the money, what the switch-off costs a business. If you want someone to audit your lines and tell you what to do, get a no-obligation quote.
What the PSTN actually is
Pick up an old-fashioned desk phone and you get a dial tone. That tone is the PSTN: a pair of copper wires from your building to the local exchange, powered from the exchange, connected through a chain of switches to any other phone in the world. It is analogue - your voice travels as an electrical signal, not data - and it is remarkably robust, which is why a copper phone still works in a power cut.
It is also old. The exchange equipment that switches these calls dates from the 1980s and 1990s, the manufacturers no longer make the parts, and the engineers who understand it are retiring. Meanwhile almost every call already spends most of its journey as data on the modern network anyway; the analogue part is a shrinking stub at each end. Maintaining a second, obsolete network for that stub makes less sense every year, which is why every developed country is retiring its PSTN, some already having done so.
What ISDN is
ISDN (Integrated Services Digital Network) was the 1980s answer to businesses wanting more than one call at a time on a line. ISDN2 carries two simultaneous calls over a copper pair; ISDN30 carries up to thirty over a fibre or copper bearer into an office phone system - the PBX. For thirty years it was how any business with more than a handful of staff got its phone lines, and it is why so many businesses have a DDI range - a block of direct-dial numbers - pointed at their PBX.
ISDN is digital, but it runs over the same exchange infrastructure as the PSTN, so it goes when the PSTN goes. It is also the more expensive part of the legacy bill, and its rental has been rising steeply as providers wind it down.
What is happening, and when
Openreach announced the retirement in 2017. Since then the programme has moved in stages: a stop-sell of new analogue and ISDN lines, exchange by exchange and then nationally, so that nobody can order a new copper phone service; then the switch-off of the remaining lines. The original December 2025 deadline was moved to give vulnerable customers and critical services more time, and the national date is now 31 January 2027. Providers are migrating customers through 2026, and many lines will be switched off well before the deadline as areas are completed.
For the customer, the practical effect is simple: you cannot buy a new copper phone line, your existing one is going, and your provider will contact you about moving. The businesses that have a bad time are the ones who wait for that contact rather than moving on their own timetable.
What replaces what
| Legacy service | What it did | Replacement | Notes |
|---|---|---|---|
| Analogue phone line (PSTN) | One phone, one call | Hosted telephony seat on an IP phone or app | Number ports across; line rental disappears |
| ISDN2 / ISDN30 into a PBX | Multiple calls into an office phone system | Hosted telephony, or SIP trunking into the existing PBX | SIP keeps a good PBX alive; hosted retires it |
| Copper broadband (ADSL/FTTC) that shares the phone line | Internet | SoGEA or full fibre (FTTP), standing alone | No phone line needed; often faster |
| Alarm signalling over a phone line | Panel dials the monitoring centre | IP or mobile (4G) signalling from the alarm company | Safety-critical; do not simply plug into a VoIP adapter |
| Lift emergency line | Lift car phone dials for help | Specialist GSM or IP unit from the lift company | Must work in a power cut; a legal requirement |
| Card terminal on a phone line | Dial-up authorisation | Broadband or mobile-data terminal | Usually a free swap from the card provider |
| Fax | Cloud fax or email | See fax after the switch-off | |
| Telecare / nurse call | Dials a monitoring centre | Digital telecare from the provider | Covered in care homes and the switch-off |
For the phones, the move is straightforward and usually saves money: seat prices are set out in what hosted telephony costs, your numbers move under Ofcom's porting rules, and the old rental disappears. For businesses whose staff already live in Microsoft 365, Teams Phone is one of the options.
Power cuts: the one thing the old network did better
A copper phone was powered from the exchange, so it worked when the lights went out. A VoIP phone needs power and an internet connection. For most businesses this is a manageable difference - staff have mobiles, and the hosted platform can route calls to them automatically - but it matters for anything safety-critical. Ofcom requires providers to offer a solution so that customers who depend on their landline can call the emergency services during a power cut, typically a battery back-up unit or a mobile fallback. Lift lines and alarms have their own rules; that is why they need specialist replacements rather than a generic adapter. The business continuity guide covers how a well-designed hosted system handles outages.
What a business should do
- List every line and what is plugged into it. Phones, alarm, lift, card machines, fax, door entry, the franking machine, the BMS. The bill helps; walking the building helps more.
- Sort the list into voice and everything else. Voice moves to hosted telephony or SIP. Everything else goes to the company that supplied the device.
- Check the connectivity. If broadband shares the phone line it will need replacing with SoGEA or FTTP; do this first, because voice depends on it.
- Choose the voice replacement and port the numbers, with the new system live before the old lines go. A good provider sequences this so nothing goes dark.
- Test the specialist devices after their replacements are installed, including in a simulated power cut for lifts and alarms.
The order matters. Businesses that start with the phones and discover the lift line in January 2027 pay emergency rates for the discovery.
The bottom line
The PSTN and ISDN are the UK's legacy analogue and digital-over-copper phone networks, and Openreach is switching both off by 31 January 2027 because the equipment is obsolete and the country has moved to an all-IP network. Voice moves to hosted telephony or SIP trunking over broadband, usually for less money, with your numbers intact. The genuine risk is the devices that depend on a copper line without anyone remembering that they do.
If you want the lines in your building audited and a replacement designed before the deadline decides the timetable for you, get a no-obligation quote or call 0333 006 9399. Our hosted telephony service page explains how the audit and quote work.
Not sure who is calling one of your numbers? Our free UK phone number checker shows which network or provider Ofcom allocated any number to, and whether the range is in service — no sign-up.
Frequently asked questions
What is the PSTN switch-off?
The retirement of the UK's traditional analogue telephone network and the ISDN services that run on it. Openreach has stopped selling new lines and is switching existing ones off, with the national deadline of 31 January 2027. Phone services move to digital (VoIP) services running over broadband.
Why is the PSTN being switched off?
The exchange equipment dates from the 1980s and 1990s, parts and expertise are running out, and almost all call traffic already travels as data on the modern network. Maintaining an obsolete analogue network alongside it is no longer viable. Most developed countries are doing the same.
What is the difference between PSTN and ISDN?
The PSTN is the analogue copper phone network - one line, one call. ISDN is a digital service over the same network that carries multiple simultaneous calls (two on ISDN2, up to thirty on ISDN30) into a business phone system. Both are being retired together.
Will my phone number change when the PSTN is switched off?
No. Your numbers port to the new digital service under Ofcom's porting rules, including whole DDI ranges. The port should be scheduled so the new system is live before the old line is ceased.
What replaces an ISDN line?
Either hosted telephony, where the phone system moves to the cloud and staff use IP phones or apps, or SIP trunking, which delivers digital lines into a PBX you already own. The choice usually depends on the age and condition of the existing PBX.
Will VoIP work in a power cut?
Not on its own - VoIP phones need power and internet. A hosted platform keeps working and can divert calls to mobiles automatically, and providers must offer a solution for customers who depend on the line for emergency calls. Safety-critical devices such as lift lines and alarms need specialist replacements with their own battery back-up.
Do I have to wait for my provider to contact me?
No, and you should not. Moving on your own timetable means you choose the provider, port your numbers cleanly and replace the specialist devices in the right order. Waiting means moving in a rush alongside everyone else in late 2026 and early 2027.