The End is Nigh for ISDN – Long Live SIP

The End is Nigh for ISDN – Long Live SIP

BT has announced that they are going to shut down their ISDN and PSTN network by 2025 and are going to move their whole network over to IP.

This is big news as all ISDN lines in the UK are from the BT network, and all ISDN and Analogue lines in the UK are connected to BT’s PSTN network.

So, everyone in the UK using ISDN will be affected, which is a lot of people. Before we go over what is going to replace ISDN, let’s go through what it is and why it is ‘failing’.

What is ISDN?

ISDN, standing for ‘Integrated Services Digital Network’ are phone lines that allow the transmission of digital data (voice) over the PSTN network. They were introduced in 1986 and were commonly used before the introduction of VoIP and SIP Trunking.

There are two types of ISDN lines, ISDN2 and ISDN30. They pretty much work the same, except for a major change in scalability.

Let’s take a closer look:


ISDN2 has a minimum installation of 2 channels, meaning 2 people can be on a call at once for each ISDN2 line you install. They are good to get if you know your business will only need at max 4 ISDN2 lines (8 users at once) for a couple years.


ISDN30 has a minimum installation of 8 channels, meaning 8 people can be on a call at once for each ISDN30 line you install. They are overall cheaper to get than ISDN2 if you need more than 8 users on the phone at once. However, in rural areas where ISDN30 is rare, ISDN2 may have to be used.

Why is it being shut down?

ISDN itself isn’t failing, and even today lots of businesses are using ISDN without problems. But, like all technology, someday it becomes obsolete.

With the introduction of VoIP and SIP Trunking, the decline of ISDN use has continued to increase. The main factors being these new technologies offering overall cheaper cost in calls and scalability, improved reliability and new features. In other words, there was a better service in town that potentially cost less than ISDN.

As these new alternatives further reduce in price and increase in availability, as expected, there will be a continuing increase in decline of ISDN use across the UK.

The question we have to ask ourselves now is what is VoIP and SIP Trunking, and why are they improved alternatives of ISDN?

What is VoIP?

VoIP (Voice over Internet Protocol) is a technology that allows you to use your internet to make voice calls, rather than through PSTN lines that a ‘traditional’ phone system would do.

What is SIP Trunking?

SIP ‘Session Initiation Protocol’ allows you to send multimedia (voice, instant messaging, video calls etc.) using the internet.

Now, this can be a bit confusing as you may have noticed that SIP allows you to make voice calls using the internet, just like VoIP does. This is because SIP would enable VoIP to happen.

Still confused? Think of VoIP as the centre of a Web Diagram. It is its own technology and there are VoIP phone systems that allow you to just make voice calls over the internet. SIP branches out from VoIP and also has its own phone systems.

Both VoIP and SIP offer cheaper calls compared to ISDN and also offer cheaper scalability. There are reliable and depending on your internet, can send out crystal clear calls. With SIP you get a host of new features that you wouldn’t get with ISDN. But, either system will arguably provide a better service than ISDN.

If you are interested in a VoIP or SIP Phone System, please don’t hesitate to call us on 0343 507 1111 or email us at to get expert advice from our telecoms analysts.

#s4s #bizfit #isdnshutdown

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