What replaces ISDN for a UK business phone system?
Two real options: swap the physical ISDN circuit for a SIP trunk and keep your existing on-site phone system, or drop the on-site system entirely and move to a hosted cloud service. Both get you off ISDN before Openreach switches it off.
Openreach stopped selling new ISDN2 and ISDN30 lines in September 2023, and the whole network they run on is being switched off on 31 January 2027. If your business runs a phone system with several lines bundled together over ISDN, that's the deadline that applies to you, same as the single-line switch-off everyone else is dealing with.
SIP trunking is the direct swap. A SIP trunk does the same job as an ISDN30 circuit: it connects your existing on-site phone system to the public phone network, so numbers, call routing and any custom PBX configuration you've already built carry over. The difference is that a SIP trunk runs over your internet connection instead of a dedicated physical line. This suits a business with a phone system worth keeping, or an IT provider who already manages it and just needs the underlying connection replaced.
A hosted cloud system replaces the PBX as well as the line. Instead of keeping on-site kit and swapping what feeds it, everyone gets a proper business number and call handling through a managed service, with no PBX to maintain on either end. This is worth considering if the on-site system is old, if nobody in the business wants to own running it, or if you'd rather buy a finished product than reconnect one you already have.
Which one is right depends on what you'd be keeping. See the fuller breakdown of what actually replaces the landline for a small UK business, and if you're weighing whether to keep running your own PBX at all, how that compares to a managed service in Olatti vs 3CX.
The full switch-off timeline, including the wholesale price rises on lines still running, is in our guide to the 2027 PSTN switch-off.
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