ISDN goes at the same time as the PSTN
ISDN2 and ISDN30 are being withdrawn as part of the same nationwide network retirement, with a final target date of 31 January 2027. There is no separate, later deadline for ISDN.
We've set out the full timeline and how the date moved twice in our guide to the PSTN switch-off date. The same phasing applies to ISDN circuits.
How ISDN differs from a standard analogue line
An analogue PSTN line carries one call at a time. ISDN was designed to carry several: ISDN2 in pairs of channels, ISDN30 in blocks of channels running into a phone system on site.
That means ISDN is typically found in businesses with a PBX, multiple extensions, direct dial numbers and defined call routing. Migration is less about replacing a line and more about reproducing how calls currently move around the business.
Stop-sell has been in force since 2023
New ISDN and analogue lines can no longer be purchased. The national stop-sell has been in place since September 2023, and in most areas existing circuits cannot be extended with additional channels either.
In practice that means growth is capped. If you need more capacity, the answer is a replacement system rather than more ISDN.
What to audit before choosing a replacement
- How many channels you actually use at peak, not how many you pay for
- Every direct dial number and number range in use, including any you've inherited
- Call routing: hunt groups, out-of-hours diverts, auto-attendant menus, voicemail boxes
- Handsets and any PBX hardware, and whether it stays or goes
- Anything else on site wired to a line — alarms, lifts, door entry, card terminals
Most ISDN30 customers find they are paying for more channels than they use. Sizing the replacement on real usage rather than the existing contract is usually where the saving comes from.
How we plan an ISDN migration
We start with the audit above, then build the new call routing on a temporary number so you can hear how it behaves before anything changes. Numbers and ranges are ported on an agreed date, with a short cutover window on the day.
Bigger estates are done in stages so a problem at one site doesn't affect the rest. Have a look at our ISDN replacement page, or send us your current bill and channel count and we'll tell you what it should look like.
Want this set up for you?
Plans start at £19 per user per month with UK calls included. See pricing or get a quote.