- ISDN stands for Integrated Services Digital Network, a digital business line technology delivered over the same copper access network as the PSTN.
- ISDN is being withdrawn alongside the PSTN as part of Openreach's all-IP migration, which is due to complete in 2027.
- The common replacements for ISDN are SIP trunking and hosted VoIP, both of which deliver business voice over a broadband or data connection.
- Openreach has been running stop-sell, after which new ISDN lines can no longer be ordered in an affected exchange area.
- Existing geographic phone numbers can usually be ported to a SIP or hosted service, so businesses can keep their published numbers.
ISDN is a digital business line technology being retired with the PSTN under the all-IP migration completing in 2027. Businesses should audit their lines and migrate to SIP trunking or hosted VoIP, keeping existing numbers.
Last reviewed: June 2026
What ISDN is and how businesses use it
Many UK organisations still run their phone systems over ISDN without realising it. Integrated Services Digital Network was introduced in the era before broadband as a way to carry several digital voice channels and data over the copper telephone network. It became the standard way to connect office telephone systems, known as PBXs, to the outside world, giving businesses multiple simultaneous lines, direct dial numbers and more reliable digital call quality than a basic analogue line could offer.
ISDN comes in two main forms. The smaller variant, often called Basic Rate, provides a handful of channels suitable for small sites, while the larger Primary Rate variant offers many more channels for busier offices and call centres. For decades this was the workhorse of UK business telephony, sitting behind reception switchboards, contact centres and multi-site networks. That long history is precisely why its retirement now requires careful planning.
Why ISDN is being switched off alongside the PSTN
ISDN runs over the same ageing copper access network as the PSTN, and it shares the same underlying exchange infrastructure. When Openreach decided to retire the legacy network and move all voice to internet protocol, ISDN was included in that programme. There is no separate reprieve for business lines: the all-IP migration covers both residential PSTN lines and business ISDN circuits, and it is due to complete in 2027.
The reasoning mirrors the wider switch-off. The equipment is end of life, parts and skills are scarce, and modern broadband can deliver the same multi-channel voice capability far more flexibly. Internet-based voice also scales up and down more easily than ISDN, where adding or removing channels traditionally meant ordering physical circuits. For businesses, the migration is therefore both a necessity and an opportunity to modernise.
It also helps to understand that ISDN and the PSTN were always two faces of the same copper estate. The PSTN served simple analogue lines, while ISDN provided digital channels for businesses that needed more capacity, but both relied on the same exchanges and the same access network maintained by Openreach. Withdrawing one without the other would have left the costly legacy infrastructure standing for the sake of a shrinking set of services. Retiring them together is what allows the underlying network to be decommissioned cleanly as the all-IP migration completes in 2027.
What replaces ISDN
The two principal successors to ISDN are SIP trunking and hosted VoIP. SIP trunking connects an existing on-site PBX to the network over an internet connection, replacing the physical ISDN circuits with virtual voice channels. This route suits businesses that have recently invested in a capable phone system and want to keep using it while removing their reliance on copper lines. It can offer flexible channel counts and resilience options such as rerouting calls automatically if a site loses connectivity.
Hosted VoIP, sometimes called cloud telephony or cloud PBX, moves the phone system itself into the provider's data centre. Handsets and softphones connect over the internet, and the call control logic lives in the cloud rather than in a box on the premises. This route removes the need to maintain on-site phone system hardware and tends to suit organisations that want predictable monthly costs and the ability to add users quickly. Both approaches deliver business-grade voice without any dependence on ISDN.
ISDN replacement options for businesses
Choosing between the options depends on the existing setup, the number of users and how much on-site hardware a business wants to retain. The table below compares the main routes available to organisations migrating away from ISDN.
| Option | How it works | Typically suits |
|---|---|---|
| SIP trunking | Connects an existing on-site PBX over broadband | Businesses keeping a recent PBX |
| Hosted VoIP | Phone system runs in the provider's cloud | Sites wanting no on-site hardware |
| Cloud PBX with apps | Softphones and mobile apps over the internet | Hybrid and remote workforces |
| Mobile-based service | Business calls routed via mobile network | Very small or field-based teams |
How to audit your ISDN lines
The first practical step is to establish exactly what is in place. Many businesses inherited their telephony years ago and no longer have clear records. A useful audit gathers the most recent phone bills, identifies the line types listed, counts the channels in use, and lists every published number along with where it currently terminates. It is worth checking the contract terms for each line, including any notice periods or minimum terms, so that migration can be timed without incurring unnecessary charges.
The audit should also catch anything beyond the obvious handsets. Door entry systems, alarm monitoring, lift emergency lines that must meet standards such as BS EN 81-28, card payment terminals and fax lines may all sit on ISDN or analogue circuits behind the main system. Each of these needs a migration plan of its own. Documenting the full inventory early prevents an overlooked device from becoming a problem when the local stop-sell or migration date arrives.
Building a migration timeline
Because the all-IP migration completes in 2027 and stop-sell is already rolling across exchange areas, there is value in planning sooner rather than later. A sensible sequence is to complete the audit, assess the suitable replacement, confirm the broadband or connectivity is robust enough to carry voice, arrange number porting, then run the new service in parallel before decommissioning the ISDN circuits. Allowing time for testing reduces the risk of disruption to inbound calls, which for many businesses are a direct source of revenue.
It is also worth engaging the existing communications provider and, where relevant, an installer early. They can confirm the migration window for the specific exchange area and advise on porting timescales for the business numbers. Leaving the move until the final months before 2027 narrows the options and concentrates demand on installers, so an earlier start generally gives a smoother transition.
Frequently Asked Questions
What is ISDN?
ISDN stands for Integrated Services Digital Network. It is a digital line technology that carries multiple voice channels and data over the copper telephone network, and it has been widely used to connect business phone systems. It comes in Basic Rate and Primary Rate variants for different sizes of organisation.
When is ISDN being switched off?
ISDN is being withdrawn alongside the PSTN as part of Openreach's all-IP migration, which is due to complete in 2027. Stop-sell of new ISDN lines has been rolling out across exchange areas. Existing lines are being migrated to internet-based voice services ahead of the final withdrawal.
What replaces ISDN for business phone systems?
The main replacements are SIP trunking, which connects an existing on-site phone system over broadband, and hosted VoIP, where the phone system runs in the provider's cloud. Both deliver multiple simultaneous calls and direct dial numbers over an internet connection rather than copper circuits. The choice depends on the existing setup and how much on-site hardware a business wants to keep.
How do I know if I have ISDN?
Phone bills usually list the line types, and references to ISDN2, Basic Rate, ISDN30 or Primary Rate indicate an ISDN service. A business with a traditional on-site PBX serving multiple extensions is also likely to be using ISDN. The communications provider can confirm exactly what is installed if the records are unclear.
Can I keep my ISDN numbers when migrating?
In most cases existing geographic phone numbers can be ported to a SIP trunking or hosted VoIP service, so a business can keep its published numbers. Porting needs to be arranged with the new provider and can take time to complete. Planning the port in advance helps avoid any gap in inbound call handling.
