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Prepare for the upcoming ISDN and PSTN switch-off


With a switch-off of existing technology on the horizon, Clarion Communications, a supplier on the CPC Communications Solutions framework, can guide CPC members through the process of switching from PSTN/ISDN to VoIP phone systems.

BT announced in 2015 that it will be switching off its PSTN and ISDN networks in 2025, transitioning fully to IP Voice services and phasing out older networks from 2023. This means that BT will cease accepting ISDN orders and that anyone looking for new or increased voice services will have to look at IP-based options for their Business Telephony Systems.

Recent surveys suggest that many businesses are still not aware of the impending switch-off and the effects that it may have on their communications. If your school uses either PSTN or ISDN lines, we recommend looking into how you can migrate your own system onto an IP service.


PSTN (Public Switched Telephone Network) – this technology allows analogue voice data to travel from one point to another through circuit-switched, copper-based phone lines.

ISDN (Integrated Services Digital Network) – allows voice and data services to be carried over digital lines. At the time this facilitated faster internet connections, multiple calls over the same line, video conferencing and DDIs (direct dial-in numbers).


As the inevitable BT Switch Off looms closer, schools and businesses across the UK must start preparing for a huge technological change. The copper-based PSTN and ISDN network has been largely unaltered since the 1980s and while to many it seems reliable, it is simply out of date. These outdated technologies that have been used by us all for over 30 years are found to no longer be able to deliver the volume of quality voice data needed in an increasingly digital world. 

At the time of the ISDN line installation, it provided a vastly improved service that allowed users to transmit not only voice data but video and other data as well. However, with many technological advances coming into our lives since the 80s, it is simply no longer up to the task.

The switch-off will inevitably impact both business and home phone telephone lines and even impact other non-voice services connected to PSTN or ISDN lines such as:

  • Security alarms
  • CCTV systems
  • EPOS machines
  • Lift alarms
  • Lines delivering Broadband services


BT’s future investment will be in a fibre-based IP network. Fibre presents businesses with two main options for replacing their existing ISDN phone lines:

SIP Trunks are virtual telephone lines over your internet connection, thereby eliminating the need for a physical phone line altogether. With SIP Trunks, organisations are no longer limited by location. As long as there is a connection to the Internet they can be set up anywhere, making them ideal for remote working from different locations. They can also be scaled up or down, enabling schools and businesses to exert greater control over call volumes and hence make cost savings.

VoIP Phone Systems allow calls to be made over an Internet connection, either with an on-premise or hosted cloud solution. VoIP systems are easily accessible via multiple devices, enabling employees to connect and communicate from desk phones, mobiles, tablets and computers. Simple to set up, cloud-based phone systems have shown themselves to be flexible, economical solutions for many schools..


If your school is still using ISDN lines, and you want to ensure you have everything in place for a smooth transition, we recommend the following:

  • Avoid a last-minute panic, and possible installation premiums, by planning for change now.
  • Check your current phone contract. Be wary about signing up to any further ISDN/PSTN obligation.
  • Evaluate and compare SIP Trunk, Hosted and VoIP options.
  • Find out if your existing Phone System already supports VoIP technology – or ask Clarion to confirm that for you.
  • Review any other business applications that run off broadband

This digital migration does mean that older lines and hardware (as mentioned above) that still rely on this technology will no longer function after the 2025 deadline. But don’t worry, there are many modern and cost-effective alternatives that schools and businesses can turn to. As long as you have a handset that can use internet protocol (IP) technology to transmit voice in a digital format using a broadband connection when you make or receive a phone call, you should be fine. This is otherwise known as VoIP.

VoIP will undoubtedly pave the way for a cheaper alternative to PSTN or ISDN for thousands of users across the UK. VoIP phone systems not only provide a more straightforward solution to analyse and track statistics of usage and other important data but also reduce the costs of these services.

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