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ISDN relies on physical cables versus, the VoIP which relies on the internet line. Traditionally, most businesses preferred to rely on an ISDN line, however, in more recent years, the stability of IP technology such as VoIP and SIP has only increased. Confidence in IP technology has grown so exponentially that BT themselves are phasing out ISDN lines which means they will be switched off by December 2025. (Learn more about the BT ISDN Switch-off.)

IP technology now seems to be the best option for businesses. Nevertheless, both, ISDN and VoIP have their pros and cons...

What is ISDN?

ISDN (Integrated Service Digital Network) transmits voice and data signals, simultaneously over digital or copper lines connected via telephone exchanges across the world.

Benefits of ISDN

  • It has been a proven technology for many years and is very reliable when it comes to the quality of voice calls
  • It is still very flexible and can be incorporated into various systems

Negatives of ISDN

  • You tend to be limited on phone numbers as they need to be geographical.
  • It can hinder growth plans as it does not easily expand to fit your business growth in terms of new users.
  • It can be costly if you do expand or move offices because of installation cost and also downtime would be an issue to overcome.
  • Limited options to keep inbound/outbound calls in service if ISDN circuit becomes unavailable
  • High rental costs for equipment
  • Some ISDN faults can take up to 72 hours to repair
  • BT are forcing a migration between April and December 2025 for all ISDN lines to then be switched off. (Read more here)

What is VoIP?

VoIP (Voice over Internet Protocol) is a fairly new piece of technology in comparison to ISDN. VoIP uses the internet to route calls rather than a physical telephony circuit.

Benefits of VoIP

  • Easily transferable if you are moving office because no physical installation is required
  • It’s easily scalable with growth with adding users being very simple and no physically installation required, too.
  • One single network for voice and data which reduces costs on line rental and improves business operations as you can use different applications such as video and I.M.
  • Flexible as you can use it to route calls to those working remotely. Also ideal in a Disaster recovery scenario.
  • Free internal calls
  • Options to choose your own phone number regardless of geographical location.
  • Low rental costs for telephony equipment

Negatives of VoIP

  • Voice quality is a concern as it is reliant on the broadband you have in place.
  • There may be an initial cost to replace other systems that you have in place as it won’t work with some systems.

Looking at options to replace ISDN?

We have a selection of resources available (for free) on the ISDN switch-off.

View all Related Resources

So, which should I choose?

The most logical solution seems to be VoIP particularly with the BT ISDN switch-off by 2025 (see Phasing out of ISDN blog.) However, ISDN still has a place until then – as long as you plan for your growth before 2023 because you won’t be able to purchase any additional lines by September 2023.

One point to note, though is that VoIP is highly dependent on the broadband quality and this can’t be ignored. For some businesses in a rural location, this just would not be a viable solution until that is addressed. For the majority, it is the best solution although you still need to consider how it will impact with the type of phone system.

Read more about “Which Phone System should I Choose?

Continue your Research on the ISDN Switch-off

We have collated a selection of free resources, purely on the ISDN Switch-off. Topics covered include:

Plus many more...
Discover our Free ISDN Switch-off Resources
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