What is PSTN? The UK Telephone Network Explained
Most business owners have never needed to know what PSTN stands for. For 150 years it has simply been "the phone network" — invisible infrastructure that made calls work. That is about to change. Openreach is switching off the PSTN in January 2027, and every business that has not migrated to a replacement technology will lose phone service on that date. This guide explains what PSTN is, why it is being turned off, and exactly what happens next for your business.
The short answer: PSTN stands for Public Switched Telephone Network — the traditional copper wire telephone network that has connected UK businesses and homes since Victorian times. Openreach is permanently switching it off in January 2027. If your business uses a standard landline, ISDN, or ADSL broadband, you are affected. The replacement is VoIP — internet-based calling that is cheaper, more flexible, and already running in two thirds of UK businesses.
What PSTN actually is
The Public Switched Telephone Network is the physical infrastructure of copper cables, exchange equipment, and switching systems that routes telephone calls across the UK. It was built in the 19th century, significantly expanded through the 20th century, and has remained largely unchanged in its core architecture since then.
When you make a call on a traditional landline, your voice is converted into an electrical signal, travels down a copper wire to your local telephone exchange, and from there is routed across the network to the recipient. ISDN — Integrated Services Digital Network — is the business-grade evolution of the same copper infrastructure, offering multiple lines on a single circuit and better call quality than early analogue services.
Both PSTN and ISDN share the same fundamental dependency: physical copper cables owned and maintained by Openreach.
Why the PSTN is being switched off
The network is failing. Ofcom reported that 2024 saw a 45% increase in PSTN-related incidents, with over 2,600 major faults recorded in 2024–25. The copper infrastructure is ageing, increasingly vulnerable to weather damage, and difficult to maintain as replacement parts for decades-old equipment become harder to source.
This is not a new problem. The telecommunications industry formally announced the retirement of PSTN and legacy networks in November 2017. What has changed is the deadline: 31 January 2027 is now confirmed as the final switch-off date, after the previous December 2025 deadline was extended to allow more time for vulnerable users and complex installations to migrate.
The switch-off is comprehensive. Openreach is not maintaining a partial legacy network alongside the new infrastructure — the copper network goes off permanently. There is no fallback, no extension, and no grandfathering of existing services.
What is still running on PSTN right now
The scale of the PSTN switch-off is easy to underestimate because the copper network supports far more than just voice calls. Ofcom identifies over 300 recognised device types that currently depend on PSTN connectivity.
Traditional landline phones (POTS) Standard analogue phone lines run directly on the PSTN copper network. Any handset plugged into a wall socket is a PSTN device.
ISDN2 and ISDN30 lines Business phone systems using ISDN circuits for multi-line calling, video conferencing, or PBX connectivity are entirely dependent on the PSTN infrastructure.
ADSL and ADSL2+ broadband Internet connections that route over the copper phone line will go offline when the PSTN is switched off. If your business internet is ADSL, it needs upgrading before the deadline.
Alarm and CCTV monitoring systems Many commercial intruder alarms, fire detection systems, and CCTV monitoring services use the PSTN to communicate with monitoring centres. These will stop working at switch-off unless migrated to IP or 4G connectivity.
PDQ card payment terminals Some older card payment terminals use the PSTN for transaction processing. Check with your payments provider — most are already IP-based, but some legacy terminals are not.
Door entry and access control systems Intercom and access control systems that dial out over a phone line are PSTN-dependent and will need upgrading.
The current state of the migration
The migration is well underway. According to Ofcom, over two thirds of UK landlines have already been upgraded to VoIP. The number of PSTN landline customers fell from 5.2 million in July 2024 to 3.2 million in July 2025 — a drop of 38% in twelve months.
That leaves approximately 3.2 million lines still on the copper network as of mid-2025. With eighteen months until the January 2027 deadline, the remaining migrations are likely to include the most complex cases: businesses with multiple ISDN circuits, alarm systems, PDQ terminals, and organisations that have deferred the decision.
For businesses that have not yet started their migration, the message from the industry is consistent: start now. The earlier you migrate, the more control you have over the timing and process. Businesses that leave it until late 2026 will find providers stretched, installation timelines extended, and the risk of disruption at the deadline significantly higher.
What replaces PSTN: VoIP and digital voice
The successor to PSTN is VoIP — Voice over Internet Protocol. Instead of routing calls over copper wires, VoIP converts voice into digital data packets and sends them over your internet connection. The call arrives at the recipient's end in the same way it always has — they hear you, you hear them — but the underlying technology is entirely different.
For businesses, VoIP arrives in two main forms:
Hosted VoIP (cloud phone system) Your phone system runs in the cloud, managed by your provider. Calls route over your broadband or leased line. You access the system via IP handsets, a softphone app on your laptop, or a mobile app. There is no on-site server to maintain. Features like voicemail-to-email, call recording, ring groups, and auto-attendant menus are all included as standard.
This is the most common migration path for UK SMEs. It replaces both the ISDN lines and the on-site PBX with a single cloud-based service.
SIP trunking If you have an existing on-site PBX that you want to retain, SIP trunking replaces your ISDN circuits with internet-based call lines while keeping your existing phone system in place. Your PBX continues managing call routing; the SIP trunks provide the connectivity to the outside world.
Both routes allow you to keep your existing phone numbers through number porting.
How much does migrating from PSTN cost?
For most businesses, moving from PSTN to VoIP reduces monthly costs immediately. ISDN lines typically cost £30–£50 per channel per month. A comparable hosted VoIP system costs £5.99–£24.99 per user per month, with significantly more features included as standard.
The migration itself need not be expensive. A competent VoIP provider handles number porting, system configuration, and testing as part of onboarding. Setup fees should be zero or minimal. What you are paying for is the ongoing service, not the migration work.
The businesses paying the highest migration costs tend to be those with complexity — multiple ISDN30 circuits, PDQ terminals, monitored alarms, and legacy PBX hardware that needs replacing. For a straightforward 10-person office on ISDN2 lines, the migration is typically complete within 10 working days.
How VoIPninjas handles PSTN migrations
VoIPninjas is a direct UK provider — not a reseller — based in Christchurch on the South Coast. We have been handling PSTN and ISDN migrations for businesses across Dorset, Hampshire, and the Channel Islands, and we offer the same service nationally.
Our approach:
- Free consultation to audit your current PSTN and ISDN services, including any non-obvious dependencies (alarm systems, PDQ terminals, door access)
- Number porting managed by our team — your existing numbers transfer to the new system without any gap in service
- Full migration support, including configuration, testing, and sign-off before we consider the job done
- 28-day rolling agreement — because we are confident enough in our service not to need a long contract to keep you
Our hosted VoIP plans start at £5.99/user/month on Ronin. Most PSTN migrations are complete within 10 working days. The free 14-day trial means you can verify call quality on your own connection before committing.
Start your PSTN migration today No tie-in, no setup fees. Most PSTN migrations are complete within 10 working days. Get a Free Quote — or call us: 0330 043 2388
Frequently asked questions
When exactly is the PSTN switch-off?
Openreach has confirmed 31 January 2027 as the date for the full switch-off of the PSTN and all ISDN services across the UK. The previous deadline of December 2025 was extended to allow more time for complex and vulnerable-user migrations. The January 2027 date is firm — there is no indication of a further extension.
Will the PSTN switch-off affect my broadband as well as my phones?
If your broadband is ADSL or ADSL2+, yes — ADSL runs over the copper phone network and will go offline when the PSTN is switched off. You will need to upgrade to FTTC, FTTP (full fibre), or a leased line before the January 2027 deadline. Most UK businesses already have access to FTTC or FTTP alternatives.
Does the PSTN switch-off affect mobile phones?
No. The PSTN switch-off only affects fixed-line copper telephone infrastructure. Mobile networks operate on entirely separate technology and are not affected.
I use a monitored alarm system — what do I need to do?
Contact your alarm monitoring company now. Monitored alarms that communicate over the PSTN will stop working at switch-off. Most alarm providers are already migrating their systems to IP or 4G connectivity, but you need to confirm your specific system is covered. Do not assume it has been upgraded — ask for written confirmation.
Can I keep my existing phone numbers when I switch to VoIP?
Yes. Number porting allows you to take your existing 01, 02, and 03 numbers to your new VoIP provider. The process takes 5–10 working days. Your provider manages the porting request — your existing lines remain active until the port completes, so there is no gap in service.
Is VoIP reliable enough to replace PSTN for a business?
Modern hosted VoIP on a stable FTTC, FTTP, or leased line connection is at least as reliable as PSTN for business calls, and typically more so. The key variable is your internet connection quality. A stable 30Mbps connection handles VoIP better than an unstable 100Mbps connection — consistency matters more than headline speed. For businesses where uptime is non-negotiable, a dedicated leased line provides the SLA guarantees that remove that variable.