ISDN2 and ISDN30
UK businesses mostly use one of two ISDN services, and the number is roughly how many simultaneous calls it carries.
| Service | Calls at once | Also known as | Typical user |
|---|---|---|---|
| ISDN2 | 2 per line (lines can be combined) | Basic rate (BRI), ISDN2e | Small offices, shops |
| ISDN30 | Up to 30 per bearer | Primary rate (PRI), ISDN30e | Larger offices, call handling teams |
What ISDN stands for, and why it mattered
Integrated Services Digital Network means voice and data sharing one digital connection. When it arrived, it gave businesses clearer calls, faster call set-up and direct dial numbers on the same lines. It also carried early video conferencing and backup data links. For years it was the standard way to connect a PBX to the outside world.
How to tell if you have ISDN
Your phone bill and your comms cabinet will usually tell you.
- Look on your bill for "ISDN2", "ISDN30" or similar line descriptions.
- Look for a small line box on the wall near the phone system, often labelled by the network provider.
- Ask your maintainer which line cards are in your phone system.
- Ask your provider for a list of every line you pay for.
ISDN versus SIP
ISDN delivers calls over dedicated copper, while a SIP trunk delivers them over your internet connection. ISDN's strength was independence from your broadband; its weakness is that capacity is fixed by the lines installed and it's being retired. SIP is easier to resize but relies on a good internet connection. See SIP trunks explained for the full comparison.
Why ISDN is being switched off
The UK's copper phone network is old and costly to maintain, so the industry is moving voice calls onto digital, internet-based networks. Openreach stopped selling new ISDN and analogue lines in September 2023, and the old network closes on 31 January 2027. Source: Openreach, 30 October 2026 Your provider may move you earlier, so check your own date.
What to do if you still have ISDN
- Find out what's on each line. Alarms and lifts can share copper services. Use the dependency checker.
- Ask if your phone system can use SIP. Some need a gateway or an upgrade.
- List your numbers. They can normally be moved, but only if nothing is missed.
- Plan the timing. If your system is also ageing, see IP Office 500 V2: support status and options for UK owners and Repair, upgrade or replace your IP Office?.
ISDN vs PSTN: what's the difference?
The PSTN is the whole traditional phone network, while ISDN is a digital service that runs over it. In everyday use, "PSTN line" usually means a basic analogue phone line, so people compare the two as line types.
| PSTN (analogue line) | ISDN | |
|---|---|---|
| Signal | Analogue | Digital |
| Calls at once | 1 per line | 2 (ISDN2) or up to 30 (ISDN30) |
| Direct numbers | Usually one | Whole ranges of DDIs |
| Typical use | Single phones, alarms, card machines | Connecting a PBX |
| Future | Being withdrawn | Being withdrawn |
How fast is ISDN?
Each ISDN channel carries 64 kbps, so ISDN2 offers 128 kbps in total and ISDN30 about 1.9 Mbps. That was quick for data once, but ISDN is now used almost entirely for voice. Speed isn't a reason to keep or replace it: the switch-off is.
What is the ISDN box on the wall?
The ISDN box is network terminating equipment: the point where the provider's line ends and your equipment begins. It's usually a small unit near your phone system, with the provider's name on it. Don't unplug or move it, because that can take down every line it serves. Note any label or line reference: your provider can use it to identify the service.
Common questions
What does ISDN stand for?
Integrated Services Digital Network: voice and data sharing one digital connection.
What is the difference between ISDN2 and ISDN30?
ISDN2 carries 2 calls per line and suits small sites. ISDN30 carries up to 30 per bearer and suits larger offices.
Is ISDN the same as a landline?
Not quite. A standard landline is analogue and carries one call; ISDN is digital and carries several. Both are being withdrawn.
What replaces ISDN?
Usually a SIP trunk or another internet-based service, with numbers normally moved across.
Is ISDN still available in the UK?
Existing lines work for now, but ISDN won't move automatically: unless you move it, expect it to stop working from 31 January 2027. Openreach has stopped selling new ones. Check your own date with your provider.
What does ISDN line rental cover?
The ongoing charge for keeping the line in place, usually separate from call charges. Check your bill and contract end date before planning a move.
Is there a way to test an ISDN line?
Engineers use dedicated line testers. The first step is to ask your provider to test the line from their side.
Sources
Last checked 9 November 2026 against the sources listed here. Menus and defaults can differ by software release, so confirm them on your own system. Spotted a mistake? Tell us.
