In 2025, telephony in the UK will change as we know it, thanks to the most fundamental upgrade to the way we make and take calls since the 1980s.


ISDN stands for Integrated Services Digital Network. ISDN uses the traditional copper telephone network for both voice and data. In 1986, this was revolutionary. You could make phone calls over your telephone line, and also transmit digital information. It paved the way for the first wave of internet adoption a few years later.
PSTN stands for Public Switched Telephone Network. It’s basically the telephone system that’s been in use since the 1880s. You might know it as the ‘landline’. People talk about the ISDN and PSTN switch-offs somewhat interchangeably, because the end result is the sam

The ISDN telephone network was originally introduced in 1986, and by 2025 it is being switched off entirely. The underlying PSTN network is already being phased out, too. Going forward, all telephony will be internet-based and so it’s critically important that businesses ensure the required level of internet connectivity is in place before addressing your phone lines.

Here at Telcom, we specialise in business internet and unified communications for organisations of all shapes and sizes. Speak to us today to find out how we can future-proof your business against the incoming ISDN switch-off.

We’ve put together some frequently asked questions about the impending switch-off which will help you and your business in deciding what action to take:

When are the PSTN/ISDN services being switched off

BT originally announced in 2015 that they will switch off their ISDN and PSTN services in 2025. These lines are being switched off by Openreach from April 2025 and all services will be switched off by the end of 2025. Importantly, before this milestone, Openreach is stopping the sale of these services.

What is the ‘stop sell’ process?

The ‘stop sell’ process is a preparatory stage before the Switch Off where Openreach stops taking new orders for these services as in a few years they are turning them off. The ‘stop sell’ is happening now from exchanges on a geographic basis depending on fibre coverage. Although ‘stop sell’ won’t affect your existing services like the Switch Off itself, this earlier milestone is significant as it means that you can’t add to or amend your existing services in their current analogue form.

I don’t use BT or Openreach - does the switch-off still affect me?

Yes – your connectivity provider uses BT and Openreach is managing and replacing the infrastructure. BT’s announcement said: “We’re moving all our customers from the old analogue ISDN telephone service to a fully digital network.” And by all our customers’ – as the infrastructure provider, they mean you and the end customers, regardless of your existing provider.

How do I know if I have analogue phones or solutions delivered via PSTN/ISDN?

Speak to your current provider or Telcom. Both will be able to advise on your lines and help you identify the devices and services running on them. Generally, the older your phones, Lift lines, EPOS, alarm systems, CCTV and other connected devices, the more likely they are to be using analogue services.

When should I act?

Now – waiting could mean that alternative solutions get more expensive. Also, organisations relying heavily on analogue services may need time to identify appropriate replacement strategies, plan the transition then migrate over to new services. Telcom can help with this strategy.

What are my options?

The two main alternatives for ISDN are VoIP and SIP. VoIP is the most well-known and popular telephone system for businesses looking to upgrade from ISDN however the choice usually comes down to which solution fits your business needs more. See telcom.uk/voice for the best, easiest, and most cost-effective way for your business to leverage the power of Unified Communications as a Service.

What are the main differences between VoIP and SIP?

VoIP is limited to carrying voice data (phone calls) over the internet, whereas a SIP trunk can transfer voice, text or video. A SIP ‘trunk’ only differs from a SIP phone in that it’s expected to carry more than one VoIP stream at once.

VoIP or Voice Over Internet Protocol makes and receives phone calls over the internet or internal networks, whereas SIP is an application layer protocol that makes, manages, and terminates multimedia sessions, including voice, video, and messaging. While SIP is an initiation, it’s mainly used to support IP technology, like VoIP calls.

Is VoIP recommended for lift lines?

The switchover from traditional telephone lines to fibre lines using voice over internet protocol (VOIP) has the potential to seriously disrupt the operation of lift alarms connected to the Public Switch Telephone Network.

Previously, providers have not recommended using VoIP to operate lift lines due to the lifts alarm devices use of Dual Tone - Multi Frequency (DTMF or “touch tone”). It is critical to check with your provider on the compatibility of these over the VoIP system if you opt to go down this route.

Luckily there is a solution readily available. - Switching to the use of a GSM network (sim card) will avoid many of the issues of switching to fibre and using VOIP. Speak to Telcom about their GSM lift line solution to find out more.

What else should I consider with critical lines?

Any alternative solution to PSTN should have a backup battery supply for a minimum of one hour duration, which is able to send an alert if the charge drops below that level. The GSM equipment provided by Telcom as part of their solution is readily battery-backed ensuring its alarm system can still work in the event of a power failure as required by BS EN 81-28 plus concerns with signal strength and availability can be addressed by using a roaming SIM which, rather than being tied to a single provider, can find and use the strongest network available;

Contact us for more information

If you have any further questions, you can discuss with our business solutions team by calling 03301 222 970 or emailing [email protected]

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About the author:
Danny Smith
SVP Telcom Networks
A get-up and go kind of guy. Secret nerd and Telcom OG since 2015 with a few grey hairs to show for it.