Future of ISDN (It’s looking a lot like SIP!)

! This post hasn't been updated in over 2 years.

Every wondered what the future of ISDN holds? This might sound surprising, but ISDN (Integrated Services Digital Network) will be gone within the next 10 years! (well, at least in the UK). BT (the UK’s main telecoms provider) has announced that the traditional network (ISDN) will become obsolete by 2025 (details can be found here: https://event.webcasts.com/viewer/event.jsp?ei=1051301, the important stuff start at about 18 minutes in). Millions of ISDN users will have to migrate to an alternative network. Ten years from now when this mass migration will have finished, all users will have been switched to IP networks and many of them will also have needed to replace or modify their current PBX.

With the recent uptake of SIP Trunking and hosted VoIP solutions, such as; Skype for Business, ShoreTel’s Connect Cloud and Gamma’s Horzion, ISDN was likely to become history sooner or later.

For many people including myself, BT’s announcement regarding the diminishing ISDN technology is welcomed. The cost and lack of function of ISDN in terms of portability, failover and quality, are among many other limitations. Now we need to identify what other options we have before 2025 arrives.

Alternative approaches to ISDN

At the moment there are really only two options; a hosted telephony solution or SIP Trunking. SIP Trunks are basically a new age ISDN line over IP, which can offer many benefits over ISDN. A SIP Trunk is a virtual link over an IP connection to your Service Provider. As the phone line is a virtual link, it has a much more versatile approach to PSTN Connectivity. Here is a basic info graphic I made for a website the other day which is a very simplistic way of looking at SIP trunks.

Future of ISDN: SIP Trunking

For more information, check out this previous blog post.

Hosted solutions is another option, where you do not have a PBX on premise any more, it is on the service provider’s network and you just have desk phones which are linked to the ‘cloud’. This may sound different to SIP Trunking, but the connectivity is pretty much the same idea, as in concurrent calls, QoS, Codecs, etc, except that in the hosted situation, the SIP Trunk is from your endpoint (phone or computer) to the hosted PBX and not from your PBX to the public phone network.

As SIP Trunking is the connectivity of your PBX to the public phone network, a direct replacement for ISDN, we will focus on that.


Benefits of SIP Trunking

SIP Trunks can be used in the existing WAN connections, which will reduce cost of additional lines. ISDN phone numbers are governed by geographic locations, phone numbers on SIP Trunks are not allocated to any specific geographic area, which provides a lot of flexibility and the possibility to consolidate SIP Trunks across multiple locations within a company as well as setting up virtual offices. Due to the utilisation of higher bandwidth, SIP is capable of transmitting HD quality voice, unlike ISDN.

In the case of any faulty lines in ISDN, the calls will be disrupted, unless the issue is resolved. This isn’t necessarily the case with SIP Trunks. The calls won’t be disturbed, they can be rerouted via another circuit or alternative endpoint depending on your network topology and service provider.

Here are a few more (not all) benefits to SIP Trunking:

  • Break the link between geographic phone numbers and geographic locations
  • Better quality than ISDN (with the right codecs)
  • Hundreds of channels on a single trunk
  • Load balance over multiple lines
  • Auto fail over
  • Quickly change the end point of the trunk (move the trunk in minutes)
  • Instantly increase a trunk for a busy period and shrink when no longer needed
  • CLI Flexibility – The ability to present any number (great for migrations)
  • Cost saving – Saving on line rental and minutes (ISDN lines are very expensive)


Before choosing a new service provider, you should first carry out research regarding the SIP services offered by the provider. Furthermore, companies should make sure the network, data centre and connectivity that the provider is offering are suitable for their needs.

Migrating to SIP Trunking

This is not as hard as you think. If you have an old PBX and you are not ready to replace it yet, you can get an SBC (session Border Controller) with an ISDN card or use an ISDN to SIP gateway and replace your ISDN with a SIP Trunk without ever letting your PBX know! A Session Border controller does many things for your SIP Trunking installation, too many to list here, so think of it as your SIP Trunking gateway. For more detailed explanation, check out this post.

If you already have an IP PBX or a TDM with SIP connectivity, then you can do a phased replacement. So you would install the SIP Trunk in parallel to the ISDN, and route outbound calls via the SIP Trunk while using CLI Flexibility so the caller is presenting their ISDN Number. Then you can start the porting process to have the ISDN numbers moved to the SIP Trunk. Once this is complete, you are done and you can cancel the ISDN. There are a couple of benefits to this approach. One, is that while waiting for the number porting you get to fully stress test the line, and you also get the outbound minute savings from the start of the project, mot once you have waiting for the number porting to complete.

So the future of ISDN…

Change is hard to accept, but businesses need to realise there are many drawbacks associated to ISDN. Although ISDN is technologically robust and one of the oldest telephony solutions, the same issues were incurred for analogue signal transmission before ISDN’s evolution. Inexpensive SIP Trunking with its array of extra features has made the general farewell of ISDN somewhat inevitable and therefore, we all should become familiar with the word ‘change’ when it comes to advances of telephony.


If you have any questions, please feel free to use the comments below.

About the author

Peter Doyle I will pretty much do anything UC! My Google+ Page My Linkedin Page


    • John Taylor on November 29, 2015 at 4:20 pm

    You might as an SBC into that nice simple diagram of yours.

    1. Thanks for the suggestion. As mentioned in the blog, this was a simple diagram made for another website, which shows a high level working of SIP Trunking connectivity. If you are interested in more detailed diagrams Of SIP Trunking and SBCs please check out my blog posts: “SIP Trunking is a Session Border Controller (SBC) needed” and “Sip Trunking 101 – The What and How of SIP Trunking

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