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  • Mathaeus Abuwa

VOIP vs ISDN - Choose telephony system

In order to make a decision about what kind of telephony system you’re going to install, you’ll need to know what each one is, and what it’s good for;


ISDN stands for Integrated Services for Digital Networks and was first defined in 1988. It has been the main business telephone solution for several years because it’s digital and of higher quality. It also allows the transmission of video and information, albeit at a much lower quality.


  • Is an established technology so has had time to improve and obtain good quality for voice calls.

  • Is more flexible as it is compatible with both established and more modern technologies.


  • Limited options to keep inbound/outbound calls in service if ISDN circuit becomes unavailable.

  • Whether you are able to keep inbound and outbound calls depends on if the circuit is already in use.

  • Numbers must be based on location, limiting the numbers your phones can have.

  • More expensive set up costs for first time installation, as well as any time the company moves premises.


VoIP or Voice over Internet Protocol is a more recent telephony solution based on the internet, creating higher quality than ISDN. It shares lines with the internet rather than needing additional infrastructure to work.

Although the technology has been around for quite a while, it only really took off in 2004 when service providers began starting up and people became more aware of it’s many advantages.


  • No set up costs as there is no installation necessary.

  • No need for separate networks for voice and data as it is all internet based.

  • Very easy to re-route calls or work from home if office connection is poor.

  • Rates are much cheaper and calls inside the business are free.

  • VoIP lines are not geographical so you can choose any number you want.

  • Other interactive communication apps can be used whilst on a voice call as both voice and data are on the same network.


  • May not be suitable for areas where the IP network is slow as this may affect the quality of the calls.

  • If you already use ISDN, it may be expensive or inconvenient to have to buy, set up and train staff to use an entirely new system.

Which one to go for?

Although this comparison may give the impression that VoIP is clearly superior to ISDN, ISDN’s main advantage is that it does not require internet connection.  This means that in areas where connectivity may be poor, or even non-existent, ISDN is really the only logical choice.

An option you can consider is having a combination of VoIP and ISDN, where the former is used as the main system for better quality, and the latter is used only when connectivity is poor.


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