ISDN Replacement for Business: Life After ISDN2 and ISDN30

Quick answer

ISDN retires with the rest of the copper network on 31 January 2027, and you have not been able to order new ISDN since 2023. Replacing it means moving your call capacity onto the internet - either SIP trunks into the phone system you already own, or a hosted phone system that replaces the on-site hardware too. Your channels become software capacity, your DDI ranges port across intact, and per-channel costs almost always fall.

The decision between those two routes is not really an ISDN question, and we answer it properly elsewhere: can you keep your existing phone system works through the keep-or-replace maths, and business landline alternatives compares all the replacement routes. What this guide does is the ISDN-specific work - reading your estate, sizing the replacement honestly, moving the numbers, and the contractual traps that only apply to ISDN. If you want your ISDN estate audited before you decide anything, get a no-obligation quote.

What ISDN is, briefly

ISDN (Integrated Services Digital Network) is the digital voice technology that has carried multiple simultaneous calls into on-site phone systems since the late 1980s. It comes in two forms you will meet in practice:

• ISDN2 - sold in pairs of channels, typically several pairs at a small site. Common wherever a small phone system needed four, six or eight simultaneous calls.
• ISDN30 - delivered as a block of channels over a single circuit, typically anywhere from 8 up to 30. This is what medium and larger sites run on.

Each channel carries one call. The technology itself is retiring alongside analogue for the same reasons, which our colleagues cover in the PSTN and ISDN switch-off explainer.

Step one: read your ISDN estate

Before anyone quotes you anything, produce this list. It takes an afternoon and it is the difference between a priced project and a guess.

The row people skip is DDI ranges. On an ISDN30 the DDI block is frequently larger than anyone remembers, includes numbers printed on documents nobody has looked at in a decade, and is the thing that has to survive intact. Start from the phone system's number table rather than from what people think is in use - see what is DDI.

Step two: size the replacement, do not copy it

The most expensive assumption in an ISDN migration is that you need the same number of channels you are renting. You almost certainly do not.

ISDN30 was sold in blocks, so a site needing eleven simultaneous calls bought a circuit configured for fifteen or twenty, and never revisited it. Nothing about the old world encouraged anyone to check, because reducing channels meant a change order and a wait.

Ask your provider for your actual peak concurrency over the last three months. Most can produce it. Then size on that figure plus sensible headroom rather than on the block you are renting. On a typical ISDN30 that exercise alone reclaims a meaningful share of the ongoing cost - and unlike ISDN, the replacement can be adjusted next month if the number was wrong. The concurrency maths itself is set out in multi-line phone systems and channels.

Worth noting: on a hosted system the model changes shape entirely. You typically buy per user rather than per channel, with concurrency handled for you, so the channel count stops being something you manage at all.

Step three: find out whether your phone system can speak SIP

This is the question the whole project turns on, and it has three possible answers.

It already speaks SIP. Systems sold in roughly the last decade generally do, sometimes needing a licence or a card enabled. In this case SIP trunks are a genuinely straightforward swap: the phone system keeps its handsets, extensions and call flows, and only the thing feeding it changes.

It can with paid work. An upgrade, a gateway or a session border controller. Sometimes worth it, sometimes the moment to stop spending on the old system - that is exactly the trade-off weighed in can you keep your existing phone system.

It cannot, or nobody knows. Out of support, undocumented, or the maintainer has stopped answering. Treat that as an answer in itself: replacing the system will cost less than prolonging it, and the migration becomes a hosted project instead.

Your maintainer can answer this in one phone call. Make that call before you talk to any telecoms provider, because the answer changes what you are buying.

Step four: numbers and routing

The numbers port; the routing does not. Both halves need planning.

• Request the full DDI range, not the numbers in use. Unused numbers in a block are still yours, and a broken block cannot be reassembled later.
• Check whether your ranges are contiguous. Sites that grew over twenty years often hold several blocks that nobody has ever listed in one place.
• Ask how porting is charged - per number or per block. On a range of a hundred this is a real difference.
• Rebuild call flows deliberately. Hunt groups, out-of-hours routing, the reception overflow that somebody configured in 2011 - none of this transfers with the numbers.
• Test every published DDI after cutover, from an outside mobile, not just the main number.

Full detail on the porting process itself, including why ports get rejected, is in keeping your business landline number.

Step five: the cutover, and the overlap that protects you

An ISDN migration is one of the few where you can genuinely run old and new in parallel, and you should.

1. Install and configure the new service while ISDN carries live traffic.
2. Test outbound on the new service using a temporary number.
3. Port a small number of DDIs first if the system allows it, and prove them.
4. Port the main number and the remaining ranges on an agreed date.
5. Run for a week with everything on the new service and the ISDN still standing.
6. Only then serve notice to cease the ISDN circuits.

Step five is the one people skip to save a month's rental, and it is the wrong trade. An ISDN30 that has been ceased cannot be reinstated - the product is not orderable. If something is wrong with the new configuration, you want the old path still there.

The two traps that are specific to ISDN

Notice periods and minimum terms. ISDN circuits are contracted services, and many carry a notice period - commonly measured in months - plus a minimum term that may have quietly renewed. Read the contract, not the bill. Two things follow: you may owe notice before you can stop paying, and if your term runs past January 2027 you should raise that with the provider now rather than discover it later. Either way, find out early enough that the notice period is not what sets your cutover date.

Cease means cease. Once an ISDN circuit is ceased, there is no route back. There is no equivalent product to re-order, and the Openreach stop-sell closed that door in 2023. Treat every cease instruction as irreversible, because it is.

For what the whole exercise typically costs, and where the money actually goes, see what the PSTN switch-off costs a business.

The bottom line

ISDN replacement is a bigger job than swapping a single line, but it is a well-understood one, and it is usually the migration with the largest ongoing saving attached - because ISDN sites are the ones renting the most idle capacity. Do the estate audit first, size on real concurrency rather than on the block you were sold, get a straight answer on whether your phone system speaks SIP, port the ranges as ranges, and leave the ISDN standing until the new service has proved itself for a week.

If you want that audit done for you - circuits, channels, DDI ranges, phone system capability and contract dates in one document - get a no-obligation quote or call 0333 006 9399.

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