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As I am sure you have heard, BT has announced it will be switching off its PSTN and ISDN networks in 2025 with the phase-out already well under way. So, now is the perfect time to start considering the move from ISDN to SIP and its benefits, on not only your bottom line, but also the extra functionality it will bring to your business.

Before we explain the time frames, process, and things to consider before the transition from ISDN to SIP, here is a quick breakdown of all the acronyms, initialisms, and information to help you make the right decision for your organisation.

What is ISDN and Analogue?

An analogue line is your old-fashioned single voice line that you have at your home which can take one call at a time with very limited functionality, these are typically still used in many small businesses.

Launched in 1986, the ISDN line was heralded for having greater capabilities - the ability to manage concurrent calls with greater voice quality and was therefore a lot more suitable for business purposes.

Back then we were all blown away by how ISDN allowed for better connections and even video communication, in a way that had not seemed possible before.

What is SIP and VoIP?

SIP stands for Session Initiation Protocol, and it is the technology that enables your voice traffic (previously carried over physical ISDN and Analogue lines) to be carried over the internet.

VoIP, is the acronym for Voice over Internet Protocol.  This is the description for voice traffic travelling via the internet. When you hear the term, VoIP phone system, it means it is compatible with SIP and has the ability to carry voice traffic over the internet. If your phone system is not SIP enabled, this then means it is only compatible with ISDN/Analogue lines and you need to look at installing a new phone system when transitioning over to SIP.

What are the benefits of moving to SIP and VoIP?

The way the SIP network has been built by the network carriers has made the management of the voice network much cheaper to house and run, thus the costs are significantly cheaper. Companies save an average of 30%, with much cheaper line rental compared to their ISDN’s as well as call rates being cheaper than either ISDN or Analogue.


One thing which is great about SIP, is that if the roads are dug up, as the council love to do on a regular basis, then you don’t have to worry about your phone lines being damaged. As SIP is carrying your voice traffic over the internet, you're not reliant on physical lines, as long as you have internet, you have voice. 

Better call quality

As well as increased resilience and cost savings, the calls you are making (as long as you have the correct connectivity) will increase in quality so your calls sound clearer and more professional.

What do I need in place before transitioning from ISDN to SIP?

Connectivity is key.  If you haven’t figured it out yet, your calls will be taking up your internet traffic, so if your current bandwidth is sufficient, then you probably need to upgrade. There are a few options:

  1. Upgrade your connectivity to a leased line, for example, to give you plenty of bandwidth for both calls and general internet usage.
  2. Upgrade your connectivity as above and apply QoS (Quality of Service) to ensure that your voice traffic has its own segregated bandwidth, and isn't impacted by any uplifts in your standard internet usage.
  3. Use a separate connectivity line for your voice. A lot of companies will simply put a separate line in, an FTTC or Leased Line depending on the number of users and call volume, to ensure that the voice traffic is unhindered.

Phone System

Ensure that your current system has VoIP capability, meaning that it has the capability to manage calls over the internet when you do transition from ISDN to SIP. If your phone system is outdated and is limited to ISDN/Analogue, then this is a great opportunity to carry out some discovery work to understand what future functionalities you would like out of your phone system in the future.

There are myriad phone systems out there, so you have to think about what you want out of a provider, how you would prefer to pay for it (monthly, one off etc.) and what functionalities you need/want (e.g. contact centre, conference calls, system integrations).

Our customers save an average of 30% on their telephony costs by switching to Intercity. If you would like to book a free audit to discuss this, then please click here.  One of our consultants will be happy to talk you through everything you need to know to be able to make the switch with confidence.

The key thing to remember is that BT are no longer going to be supporting ISDN, so the quicker you switch before the deadline of 2025, the quicker you can start benefiting from those cost savings and added functionality.


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This content was about: Unified Communications, ISDN Switch off

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