The Big Switch Off: an introduction

The Public Switched Telephone Network (PSTN) has been the basis of the UK’s telephony infrastructure since the first telephone lines were introduced in 1875.

It was introduced to eliminate the need for manual call handling. The PSTN has long been a widely accepted and reliable standard for communication due to its extensive nationwide coverage. However, over the past decade, it has experienced a consistent decline in usage.


The PSTN supports several connectivity products that are sold to businesses in the UK:

  • PSTN-enabled landlines, like PBX phone systems
  • Integrated Services Digital Network (ISDN) lines — these could be ISDN2 or ISDN30 lines
  • Asymmetric Digital Subscriber Line (ADSL) broadband connections


BT Openreach, which operates the telephony infrastructure in the UK, has decided to retire the PSTN. This means the system will be entirely switched off in December 2025. If your business communication relies on any of these technologies, you’ll need to adopt a new system before the big switch off.

PSTN switch off

How does the PSTN work?

The PSTN encompasses an extensive network of telephone ex-changes and copper wire transmission lines designed for analogue signal transmission.

Individual subscriber fixec lines connect to local exchanges, which, in turn, establish communication with trunk exchanges, main hubs, and central exchanges. Typically, lines within a local exchange share the same area code. To call outside this area, it’s essential to include the area code.

The PSTN also supports the Integrated Services Digital Network (ISDN). The ISDN emerged as a solution for digital data and voice transmission over regular phonelines by offering superior voice quality compared to the PSTN, and can integrate both speech and data on a single line. This enabled users to make faster calls, as well as take and receive video calls too.

Why is the PSTN being switched off?

    The PSTN consists of hardwired copper lines that connect individual phone lines to the telephone exchange. The same copper connections have been in place since the PSTN’s introduction in 1875.

    There are several reasons why the PSTN is being switched off:

    • Technological advancements — newer digital communication technologies enabling voice communication provide more efficient and cost-effective services and aren’t supported by the PSTN
    • Maintenance costs — the equipment that makes up the network is reaching its end of life. Maintaining the network is becoming increasingly challenging, and the cost implications are outweighing the benefits of keeping the system functioning
    • Decreased usage — as more people switch to digital and mobile services, traditional landlines are less frequently used


    Therefore, BT Openreach has decided to gradually retire the PSTN and services that it supports. Instead, all homes and businesses will need to switch to new, digital alternatives for their voice services and internet connections.




    PSTN switch off timeline

    • 2018 — Openreach announced retirement of PSTN and ISDN in 2025
    • December 2020 — five-year reminder that the switch off is approaching
    • September 2023 — the ‘stop sell’ covers the entire UK to cease all sales of PSTN services. This means it’s no longer possible to purchase a PSTN or ISDN phoneline, or even make changes to an existing connection
    • December 31, 2025 — target date for complete withdrawal of all PSTN and ISDN services from the UK
    • January 1, 2026 — all phone calls will be hosted via the internet

    What is replacing the PSTN?

    Voice over Internet Protocol or VoIP is the main technology that will replace the PSTN for voice calls. Learn more about the alternatives to the PSTN here.