If your business runs an on-site phone system, there's a good chance it sits on ISDN - and ISDN is being switched off along with the rest of the old network. Replacing it is more involved than swapping a single line, so here's what ISDN2 and ISDN30 replacement actually looks like.
A quick reminder: what ISDN is
ISDN (Integrated Services Digital Network) is the older digital phone technology many businesses use to run multiple simultaneous calls into an on-site phone system (a PBX). It comes in two main flavours:
- ISDN2 - lines in pairs of channels, common for smaller setups.
- ISDN30 - delivered in larger blocks of channels (typically from 8 up to 30), used by bigger phone systems.
Each "channel" is one simultaneous call. We explain lines and channels in multi-line phone systems.
Why ISDN has to go
ISDN runs on the same copper infrastructure as the analogue PSTN, and it's being switched off on the same timeline - 31 January 2027. New ISDN orders stopped back in 2023, and legacy charges are rising through 2026. So if you're on ISDN, replacement isn't optional.
Your two replacement routes
The right path depends on whether you want to keep your existing phone system.
1. Keep your PBX with SIP trunks
If your on-site phone system is relatively modern and you're not ready to replace it, SIP trunks can deliver your calls over the internet straight into it - effectively a like-for-like replacement for ISDN channels. We compare this with the alternative in SIP trunking vs hosted VoIP.
- Best for: businesses with a capable existing PBX they want to keep for now.
- Bonus: usually cheaper per channel than ISDN, and easier to scale.
2. Move to a hosted phone system
The fuller move is to retire the on-site PBX entirely and go to hosted telephony, where the "system" lives in the cloud.
- Best for: businesses wanting to modernise, support remote working, and drop on-site hardware.
- Bonus: features like auto-attendant, call recording and analytics come built in.
For help deciding, see hosted vs on-premise PBX.
Keeping your numbers and DDIs
A big worry with ISDN replacement is the number ranges - especially DDI blocks where staff have direct numbers. The reassurance: these can be ported to the new service, including DDI ranges, so nobody loses their direct line. See keeping your landline number.
Planning the migration
ISDN replacement touches your whole phone setup, so plan it properly:
- Document your current setup - channels, numbers, DDIs, and how calls route.
- Check your connectivity - enough bandwidth and ideally resilience; see internet failover.
- Choose SIP trunks or hosted based on whether you keep the PBX.
- Port numbers and test before switching off the old lines.
Our switch-off checklist keeps this in order.
The bottom line
ISDN is on the way out, but replacing it is an opportunity to cut per-channel costs and modernise. Keep your PBX with SIP trunks, or move fully to hosted telephony - either way your numbers, DDIs and call capacity come with you. Want a migration plan for your system? Explore our Cloud Telephony service or request a callback.
Frequently asked questions
When is ISDN being switched off?
ISDN is being retired alongside the PSTN on 31 January 2027. New ISDN lines could no longer be ordered from 2023, and legacy charges are rising through 2026.
What replaces ISDN for business?
Internet-based voice. You can either keep a capable existing PBX and feed it with SIP trunks, or move to a fully hosted phone system in the cloud. Both replace ISDN channels with more flexible, usually cheaper capacity.
Will I lose my DDI numbers when I replace ISDN?
No. Your main numbers and DDI ranges can be ported to the new service, so individual direct-dial numbers are preserved.
Is SIP trunking or hosted telephony better for replacing ISDN?
SIP trunking suits businesses that want to keep their existing PBX, while hosted telephony suits those ready to retire on-site hardware and modernise. The best choice depends on the age and capability of your current system.
