What Is ISDN in Networking? Meaning & Definition

ISDN stands for Integrated Services Digital Network. It is a set of standards for sending voice and data as digital signals over the old copper telephone network. Put simply: ISDN made it possible to make several phone calls and send data over one line at the same time.

That was a big deal in the 1980s and 1990s. Today, it is a different story. Openreach is switching off ISDN in the UK, and the final date is 31 January 2027. So if your office phones still run on an ISDN line, now is a good moment to ask: what is ISDN doing for you, and what will you use instead?

 

What is ISDN in networking?

To understand ISDN in networking, it helps to look at what came before it. Old analogue phone lines carried one call at a time as an electrical signal. Noise, crackles and dropped calls were normal.

ISDN changed that. It turned voice into digital data and sent it over the same copper wires. This gave you:

  • clearer call quality
  • faster call setup
  • more than one call on a single connection
  • the option to send data, like a fax or an early internet connection, next to your calls

ISDN uses circuit switching. When you make a call, the network reserves a fixed channel for you from start to finish. That channel stays yours until you hang up, even when nobody is talking. It is reliable, but it is not very efficient. Modern Voice over IP shares the connection much more cleverly.

 

What is an ISDN line?

An ISDN line is a physical phone connection that carries digital channels instead of one analogue signal. Each channel can carry one phone call. In the UK, you will usually come across two types of ISDN lines.

ISDN2 (Basic Rate Interface)
This gives you two channels for calls or data. It is a good fit for small offices that need two calls at once.

ISDN30 (Primary Rate Interface)
This gives you up to 30 channels over one connection. Larger offices, call centres and organisations with an on site phone system often use ISDN30.

Picture a busy dental practice. Two receptionists are on the phone and a third call comes in. With ISDN2, that caller hears a busy tone. With ISDN30, there is room for the call, but you pay for every channel, whether you use it or not.

 

The definition: ISDN in plain English

The ISDN meaning is simpler than the jargon suggests. If you want a short version to remember, try this: ISDN is a digital phone line that lets you make several calls at once over copper wire.

A few terms you may see:

  • Integrated services digital network: the full name. “Integrated services” means voice and data travel over the same line.
  • Channels: each channel carries one call. More channels means more calls at the same time.
  • Circuit switching: the network keeps a fixed route open for each call.

 

Why ISDN is switching off

ISDN did its job for decades. But the copper network behind it is old and expensive to maintain. Openreach and the wider UK telecoms industry are moving all phone traffic to internet based connections.

Here is what that means for you:

  • You can no longer order a new ISDN line. The stop sell is already in place.
  • Existing ISDN lines will stop working after the switch off on 31 January 2027.
  • Any phone system that relies on ISDN needs a new connection before that date.

Waiting until the last few weeks is risky. Migration takes planning, especially if you have several sites, call queues or hunt groups to move.

 

What replaces ISDN?

The replacement for ISDN is internet based calling, also known as VoIP or cloud telephony. Instead of a fixed copper channel, your calls travel over your internet connection. If you want to keep your current on site phone system for a while, a SIP trunk can replace the ISDN connection in the meantime.

With Xelion, your phone system runs in the cloud. You don’t need a phone system box in the comms cupboard, and you don’t pay for a fixed number of ISDN channels. Here is what that looks like day to day:

  • A customer calls. You see who it is and who on your team is available to answer.
  • It gets busy. Call queues and hunt groups spread calls across your team, so callers don’t hear a busy tone.
  • You work from home. You make and take calls on your laptop or smartphone with the same business number.
  • You need more lines. You add a user in the settings. No engineer visit, no new channels to order.
  • You want insight. Reports show how many calls came in, how many you missed and when your busiest moments are.

Everything sits in one licence, so you don’t buy separate modules for telephony features like call recording or the wallboard. In most cases, you can also keep your existing phone numbers when you move.

Xelion works with a network of UK partners. Your partner plans the move with you, sets everything up and stays your point of contact afterwards.

Frequently asked questions

ISDN stands for Integrated Services Digital Network. It is a digital phone line that carries voice and data over copper telephone wires.

Businesses mainly use ISDN for phone calls, especially with an on site phone system. In the past, people also used it for fax and early internet access.

It is the end of ISDN and analogue phone lines in the UK. Openreach will switch off the old network on 31 January 2027. After that, ISDN lines stop working. You can read the latest Openreach update on ISPreview.

No. New ISDN lines are no longer available. If you need extra lines now, cloud telephony is the way to go.

In most cases, no. You can usually move your existing numbers to your new cloud phone system.

Not always. Many IP desk phones work with Xelion. You can also use the Xelion apps on your computer or smartphone.

FC_Xelion_Figuur_POSE_03_vrijstaand.png

Ready to say goodbye to ISDN?

The switch off date is getting close. Moving now gives you time to plan, test and train your team without stress. Want to see how Xelion works in practice? Request a demo or contact us. We’ll connect you with a Xelion partner who can help you make the move.

Request a demo Contact us