ISDN2, ISDN30 and PRI explained (and why they're going away)

If someone has ever told you that your business has “a PRI” or “an ISDN30”, and you nodded along without knowing what that meant, this is for you.

Start with the box in the cupboard

Any business with more than one phone and the ability to transfer calls has a PBX, a Private Branch Exchange. Think of it as a miniature phone exchange sitting inside your building, separate from the national phone network.

Traditionally that was a physical box of electronics in a comms cupboard. Every desk phone had a cable running back to it. The box decided which phone rang, let people transfer calls, and played the hold music.

Each desk phone got a short internal number, an extension, like 201 or 305. That is why, when you ring a company and get transferred, someone says “I’ll put you through to extension 214”.

How calls got in and out

The PBX on its own could not reach the outside world. It needed lines running back to the phone network, called trunk lines.

Two words matter here:

• A bearer is the physical line itself, the actual cable coming into the building.
• A channel is a slot on that bearer that can carry one call.

How many channels a bearer carries depends on the technology. That is the entire difference between the three types below.

What you have	Bearer	Channels	Calls at once
POTS (a plain analogue line)	One	One	One
ISDN2, also called BRI	One	Two	Two
ISDN30, also called PRI	One	Thirty	Thirty

A small business might have two or three trunk lines shared across twenty extensions, on the reasonable assumption that not everyone is on the phone at once. That is also why an office phone system can only take so many outside calls before callers get an engaged tone. The trunk lines are the bottleneck.

Why the number thirty is fixed

This is the bit worth understanding, because it is where the money goes.

On an ISDN30, thirty channels is a hardware fact. The circuit is built that way. If your business grows and you need more, you order a new physical circuit, wait for it to be installed, and pay for it. If your business shrinks and you only use eight, you still pay for thirty.

There is no dial to turn. It is thirty because the cable says thirty.

What replaces it

A SIP trunk does the same job, but it runs over your existing internet connection rather than a dedicated physical line. SIP just means Session Initiation Protocol, the technical standard used to set up an internet call.

The internet connection becomes the bearer, and the channels on it are virtual. Configured in software, not fixed by how many wires are in a cable.

The practical difference:

• On a PRI, thirty channels, take it or leave it, new circuit required to change.
• On a SIP trunk, five, ten, twenty, fifty, whatever you actually use. Changed remotely, usually the same day, no engineer and no new cable.

If you are on an ISDN30 right now, there is a decent chance you are paying for capacity you have never touched. Worth checking your actual peak concurrent calls before anyone quotes you a replacement.

Why this is now urgent

All of the above runs on the PSTN, the copper phone network that has existed in the UK for decades. It is being switched off on 31 January 2027.

POTS, ISDN2 and ISDN30 all go. Not “become unsupported” or “get more expensive”, although both of those are happening on the way. They stop.

The full switch-off explanation is here. If you want the short version of where your business stands, the switch-off checker does it in about thirty seconds.

What to do about it

Find out what you have. Your bill will usually say, or an old invoice will mention ISDN. If you cannot tell, describe what is in the cupboard to someone who can.

Then find out what you actually use, not what you are paying for. The gap between those two numbers is usually what makes the move pay for itself. Our pricing is here if you want a figure to compare against.