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The big PSTN and ISDN switch off – what does it mean for you?

6 March 2023 Time to read:  minutes

In December 2025, the BT Openreach Public Switch Telephone Network (PSTN) will be switched off. This means that the old telephone network, that has been used since the last century, will no longer be available. PSTN and ISDN simply don’t support the speed, scalability, and methods of communication that businesses use today. Therefore, BT Openreach has decided to stop investing in a technology that can’t fulfil these requirements.

Pstn Small Img

By 2025, every phone line in the UK will need to move to a fully digital network that uses Internet Protocol (IP) across a fibre-based service. Do you have a plan for your business? If not, then read on for our advice on how you can prepare now for the PSTN and ISDN switch-off.

Who’s affected by the PSTN and ISDN switch-off?

It’s not just businesses affected by the closure announcement, but also Public Sector entities, NHS bodies, education institutions and charities.

The services that will be affected are:

  • Broadband using a telephone line (ADSL)
  • ISDN2 and ISDN30 for business phone systems
  • Analogue lines including:
  • Alarm lines
    – CCTV
    – Patient care lines
    – Door entry systems
    – EPOS and card machines
    – Remote telemetry systems
    – Fax machines
    – Lift alarms

What is happening and when?

You may not realise the switch-off will have an impact on you this year. In September 2023 you won’t be able to purchase new ISDN lines – the clock is ticking.

Bt Openreach Timeline

What must I do to mitigate the impact on my business?

All businesses currently using PSTN and ISDN services will need to migrate to a new service as the current infrastructure will simply stop working.
You need to plan how you will mitigate the impact of this change on your communication strategy. How will you communicate internally and externally, with clients and service users, suppliers and partners in the near future?

This provides an ideal opportunity for you to streamline operations, drive cost efficiencies and improve customer service.

Our five-point best practices plan gives you a simple framework to ensure you mitigate the impact on your business and optimise for the future:

  1. Audit your current estate
  2. Understand your contractual position
  3. Define your telephony strategy
  4. Access the opportunity
  5. Engage with trusted telephony partners

You cannot do this alone – Trustmarque is here to help you plan and execute a new communication strategy.

How can Trustmarque help?

Our experienced consultants will work with you to plan the most effective deployment to meet your needs. We provide services to ensure a smooth transition away from your legacy PSTN and ISDN network:

  • End-to-end consult, deploy and migrate, onboard, and support professional services
  • AutoDiscovery service to accelerate and de-risk transition from your current technology
  • Implementation and staff training services to ensure rapid user adoption and return on investment

The BT Openreach PTSN and ISDN switch-off is the perfect opportunity to realise the benefits of a modern telephony infrastructure:

  • Cost control and predictability
  • Scalability
  • Business continuity
  • Operational efficiency

What are my Options?

Trustmarque has many years of experience in consulting and delivering voice services to our customers. Our services include:

We work with a range of partners to deliver choices accompanied by professional services to help you select the right solution and implement it successfully.

Please contact us or get in touch with your Trustmarque Account Manager and start your transition to an all-IP solution.

The big PSTN switch off - what does it mean for you?

On demand webinar

If you are still weighing the pros and cons of investing in Voice over Internet Protocol (VoIP) or Session Initiation Protocol (SIP) Trunking technology, the time to act is fast approaching. In December 2025, the BT Openreach public switch telephone network (PSTN) will be switched off. Watch our on-demand webinar to find out more.


  • What is the 2025 PSTN switch-off
  • The impact on organisations
  • Five-point best practice plan
  •  How you can mitigate the risks of  the PSTN switch-off
  • User cases

During this webinar hosted in partnership with Gamma, Trustmarque’s Voice Network services team will explain how this is an opportunity to streamline operations, drive cost efficiencies, and improve your customer service and engagement.

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