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

October 7, 2021 Claranet Limited

Alex Dunlop
Project Manager at Claranet

In the first of these posts, I looked back at the announcement that concerned the end of PSTN and ISDN voice networks, cornerstones of the UK’s communications for over a century, are to be retired.

The headlines from that announcement are:

  • 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.

In this post, I want to consider what the immediate future holds for our communication services and the people and businesses that consume them, up to the big switch off in December 2025.

I think we can expect sharply different experiences for the business and residential users of the networks, which will manifest in the approaches of suppliers to those communities.


UK Businesses have been targeted for some years now with alternative technologies to replace their ISDN connections, whether that’s SIP Trunking or fully hosted services like Claranet’s Hosted Voice and Teams Voice services. Channel volumes for ISDN30 are now below 2 million, ISDN2 channel volumes are now below 0.5million.

Those efforts will doubtless be redoubled, and we have also seen some suppliers ramping up their rhetoric, and pulling the End Of Life date a bit closer to try and add impetus and urgency to organisations that have been unreceptive until now.

Even putting that aside, organisations with large and complex telephony estates, especially ones with nuanced PSTN setups will need to start planning what they are going to do. It won’t be possible to copy/paste setups using PSTN Multi-Line, diverts and AUX lines into the new world of hosted telephony, so organisations will need to break down their site requirements and work with suppliers like Claranet to design new solutions for stores and small office networks.

On a more practical level, suppliers will now have to stop offering long contracts to customers – 3 or 5-year contracts will be impossible as there will be no ability to supply new lines after 2023, so viable new contract lengths will shorten as time goes on. Anyone left with PSTN and ISDN services in 2023 will have a very limited set of options as to what they can do with them – some modify journeys will remain but not many.

Also, remember that Carrier Pre Selection (CPS) is also going End of Life at the same time, so contracts for call usage will also shorten and become harder to get.

I suspect some organisations will start to stop offering traditional voice services to their customers, and refusing to renew contracts for them. Nobody will want to be stuck trying to migrate a complex customer PSTN network while the clock ticks down.
At the same time, organisations will be being offered the new range of ‘Single Order’ broadband services, to replace their existing data connections. There are few of these already on the market, such as Fibre-To-The-Premise. A few customers of these have already found that despite buying a ‘broadband line’, you cannot plug a phone into the wall socket! All these new services will have that same experience, and doubtless a few more people will be surprised that they haven’t got any telephone service there.

It should be mentioned at this point that there is one group of suppliers and customers who aren’t necessarily changing. ‘Full LLU’ operators like Talk Talk have been supplying their own voice service over copper lines into their unbundled exchanges for some time, and these services are NOT included in the End of Life and End of Sale notices.

The message for businesses and organisations at this time should be to start planning, to look at their estates and identify where they have PSTN and ISDN services and start to consider what to do about it. In many cases the alternative service, such as Claranet Hosted Voice and Teams Voice, is already available, and normally offer substantial business benefits vs traditional PSTN/ISDN anyway. There is nothing to be gained by waiting for the ‘last orders’ bell to ring before starting to move!


The picture for residential consumers in the coming years is also complex, and while little has been said about the plans/options, I think we can expect to see some of the following hit the market, and the news, in that time.

We should shortly see the first of the ‘broadband only’ or ‘no line rental’ products come to the residential market, offering a cost-effective data-only connection. Many consumers have in effect migrated to mobile for their voice calls, so the ‘home landline’ has been gathering dust for a while now. Suppliers will target and seek to grow that segment.

As mentioned above, customers of ‘Full LLU’ providers like Talk Talk may not have any change forced upon them, though the pressure on prices and bandwidth provision from the newer services may mean that even they seek to move customers onto newer services to retain them.

For the remainder who are on PSTN lines with or without broadband, the options are unclear. They are even more unclear for services that utilise the PSTN for dial-up connectivity like lifts, Franking machines, PDQ machines, alarm lines, dial up data for Sky boxes, and the many other legacy uses PSTN lines have been put to.

BT have established a test and development environment for suppliers of these services to be able to test new approaches. Some will migrate to IP, some to other means like GSM mobile units with battery backup, to retain connectivity even in the event of power failure.

As time ticks on, domestic suppliers will have to offer their customers the option of migrating to the new ‘all data’ lines, and bundle a voice service as an overlay, to replace the PSTN service. These are very likely to involve new on-site hardware with analogue-to-digital converters in domestic routers to allow older handsets and equipment to be connected, plus a new range ‘VoIP’ home phones, which are likely to favour WiFi to reach distant parts of buildings. The number and the phone service itself will then be ‘ported’ from the PSTN to the overlay hosted VoIP service, hopefully with the minimum of disruption.

I can foresee a time soon when those of us in the industry will be called upon by our friends and family to explain what’s going on! Better get the slideware ready for that one.

Inevitably, come 2023 and then come 2025 there will be a remnant of customers both business and residential who haven’t been able to move or are unwilling to. It remains to be seen whether Openreach will proceed with their switch off plans if that remnant is still sizeable – cutting off people’s phone lines in bulk is the sort of thing that makes the news, so I would also expect the marketing effort from Openreach and from the various domestic and business suppliers in the market to get louder and louder as the dates come closer.

In the last of these posts, I’ll look ahead to the post-2025 world to see what our future communications market looks like in the UK.

Future scope: the shape of communications post-2025

Previous Video
Claranet | It's Your Call
Claranet | It's Your Call

Reasons to dial into Hosted Voice.

Next Presentation
Moving on up: how agile telephony makes for a better business
Moving on up: how agile telephony makes for a better business

Missed calls can lose businesses surprisingly large amounts of money. They’re bad for morale and customer r...