As of 2025,  BT will be switching off its PSTN and ISDN networks, fully transitioning the UK’s telecoms to IP Voice services (VoIP) and phasing out older networks from 2023. This means that BT will cease accepting ISDN orders and that any businesses 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 business communications. If your business uses either PSTN or ISDN lines, we recommend looking into how you can migrate your own system onto an IP service.

Fear not, there is a great deal of positivity. Firstly, using IP and Cloud Telephony has significant cost benefits, reducing or eradicating line and call costs. Provisioning is much quicker and more easily controllable and service can be accessed from anywhere on multiple devices, making your business more agile and responsive. And there are a plethora of new features and functions available.

What Is PSTN?

PSTN (Public Switched Telephone Network) is the traditional circuit-switched telephone network that transmits analogue voice data across copper-based circuit-switched phone lines. The PSTN is operated by local, national or international carriers.

What Is an ISDN line?

ISDN stands for Integrated Services Digital Network, offering voice and data over digital lines via further services such as DDIs (direct dial-in numbers) and Video Conferencing. And allows for multiple calls on different channels over the same line.

What is VoIP telephony?

VoIP (Voice Over Internet Protocol) telephony allows you to manage calls, text, and video meetings using an internet connection rather than traditional telephone lines. Sometimes VoIP can be referred to as internet protocol, IP, telephony or cloud-hosted telephony.

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    PSTN and ISDN are legacy technologies that have been significantly upgraded over their life-time but have high maintenance and running costs. BT is looking to switch to a single IP Network, allowing for SIP and VoIP, which offer multiple benefits to businesses. Investment in PSTN and ISDN lines is ceasing while there is ongoing investment in the IP Network.


    SIP (Session Initiation Protocol) provides a phone line over a fibre internet connection.

    VoIP is cloud based telephony whereby your phone calls are sent over your internet connection and your phone system is moved ‘online’. You can access your system via multiple devices – deskphone, mobile and tablet and it offers many additional features such as call recording, voicemail to email and unified communication benefits.


    If you would like find out more about making the switch and the financial and operational benefits it can bring, contact our Team of expert Advisers on 0333 222 6633 or email us on

    Clarion Communication ManagementWHAT YOU NEED TO CONSIDER:

    Find out your current contractual obligations and think very carefully before renewing any ISDN or PSTN contracts.

    Look at the options available. Many newer Phone Systems support IP Telephony. If yours doesn’t, you will need to replace the system with a VoIP system or an IP-enabled on-premise PBX (phone system)

    When switching to SIP/VoIP based systems, you will need to make sure that you have sufficient bandwidth to deliver voice on top of the existing services that your office data line supports, otherwise you may experience poor voice quality or intermittent service.


    • Massive Savings –  removal of line rental charges, free UK Landline and Mobile calls and between your organisation’s sites and much reduced Intl charges.
    • Flexible Working – Log into your Phone System from anywhere and any device with an internet connection, and take your number with you.
    • Advanced Functions & Features – incl video conferencing, CRM and email integration, analytics and increased control etc.
    • Scalability – Add, remove or reassign lines swiftly according to your changing business needs.
    • Greater Resiliency – Automatic back-up and failover to keep you connected.
    • Swift Deployment – No more engineer-led physical installation so you can get connected faster.

    MAKE THE SWITCH TODAY – If you would like find out more about making the switch and the financial and operational benefits it can bring, contact our Team of expert Advisers on 0333 222 6633 or email us on


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      Louise Sanderson Head of Sourcing - Simon Jersey
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