100 years in the making: the end of PSTN and ISDN networks

October 7, 2021 Claranet Limited


Alex Dunlop
Project Manager at Claranet

Recently we have seen what I believe is the most significant announcement on the UK telecoms market in a generation, the retirement of the PSTN and ISDN networks.

In this short series of posts, I’d like to take that moment to consider what this means, what we know so far about this change, and, looking forward, what our market is going look like in the near future.

So, in the spirit of looking back, and also for people who may not fully understand how momentous this decision is in a world of WhatsApp and Twitter, let’s consider the history of the telephone ‘line’ and how we got to where we are today.

Since Victorian times, the UK’s telephone service has utilised copper cables (usually two lines, in a ‘copper pair’) to connect telephones back through telegraph poles and street cabinets to local telephone ‘exchanges’, linked together to create a national telephone network, or PSTN. Originally under the auspices of the GPO (General Post Office) and after 1981, British Telecommunications, then after 2006, Openreach, the copper line and telephone exchange network has connected homes and businesses all over the UK for well over 100 years. The familiar socket in the walls of our homes, the shape and type of telephone handset connected into that network, the user experience that generations have grown up with, and the physical infrastructure behind those, haven’t changed much in all that time.

The parallel development of ISDN, a multi-channel PSTN connections that allowed several calls (up to 30) to take place on the same line at the same time, also allowed businesses to buy telephony services in bulk, and many organisations still do.

The most significant change in recent years was the development, in the 1980’s and 1990’s, of DSL technology, which allowed digital data to use the same line as the telephone communication, without interfering with the voice call. This neat technical trick eventually resulted in the wide adoption of DSL Broadband across the UK, breathing new life into the copper lines and local telephone exchanges.

In all that time the underlying voice telephony service has been in place. The consistent experience has been that if you plug a phone into the wall socket, you will get a dial tone and be able to place a call, unless you have taken the step to put call barring in place. Even if the line has never had a phone connected to it, that capability has never been removable.

For the last 30 years or so, the situation has been relatively settled. People have continued to buy copper lines under a variety of nomenclatures, for their homes and businesses, delivering the same user experience of dial tone and numbering, broadband and calls.

However, in the early summer of 2018, Openreach (no longer a part of BT) announced a series of presentations around the UK and invited the telephony reseller community for both residential and business customers, to hear their statement on the future of their service.

Most of what they presented was well known to their audience. Use of the PSTN fixed line telephony network was declining quickly, dropping by half in the last 6 years alone (https://www.theguardian.com/business/2019/jan/05/britons-hang-up-landline-call-volumes-halve). This has had the effect of sharpening minds in Openreach and making hard economic choices more urgent. That voice network, provided now by Openreach on the skeleton provided by BT and the GPO before them, was no longer paying for itself as the volume of calls dropped. Also, the costs of maintaining the older technology in telephone exchanges is rising, or in some cases the equipment could no longer be maintained as replacements were no longer being made.

Openreach therefore announced two target milestones to the industry.

From 2023, all PSTN and ISDN voice services would go End of Sale.

From 2025, the PSTN and ISDN voice networks would be turned off.


At the top level, what this is going to mean is that at the end of 2025, the familiar phone-in-wall-socket telephone voice experience is going to come to an end, after over 100 years. For everyone – not just businesses, who have been moving steadily to voice-over-IP (VoIP) services for some years now, but also for the millions of domestic users.

All Openreach connections from 2025 will be ‘data only’ – all broadband. Voice services will only be available as Voice over IP service provider based offerings that run over the top.

It should be noted that in the months since these announcements, Openreach have not wavered from this timescale, and we therefore must work on the assumption that these dates will be hit.

In the next of these posts, we’ll look at what this means for the market as it stands today and in the final post we’ll consider what the market might look like after 2025.  

The big switch off: how the transition from PSTN and ISDN may play out

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