Businesses usually use digital or analogue telephone lines for both receiving and making calls. The most commonly used virtual line that has replaced these types of methods is SIP trunking, as it allows businesses to connect all kinds of voice calls via the internet, which allows more flexibility and security. The question remains, what is ISDN and what does SIP accomplish. Learn about these and how they differ ahead.


ISDN provides you with a physical connection as opposed to a wireless type. A BT engineer brings this into your building through a BT exchange along with a small box or with a few small boxes that are installed all throughout your building. These are all physically connected throughout the building. Every ISDN30 circuit consists of a total of 30 trunks/channels and is one box; additionally, every ISDN2 circuit includes two trunks/channels in a single box. As an example, an eight channel ISDN2 would consist of a total of 4 small boxes for installation to work correctly.

SIP trunking is different as the connections are virtual. You can mostly choose how many trunks you have, but there will be a few restrictions. Restrictions are based on the total bandwidth you have and the overall data connection speeds available, but one thing that is certain is the fact that no physical termination hardware is required inside the building.


When you choose to scale up the ISDN needs the installation of new circuits along with more ISDN boxes. ISDN2 allows for an upgrade of increments of 2 channels/trunks at once. Now when it comes to ISDN30, there is a total of 30 channels in the pipe that are installed, and what is great is that anywhere from 8 to 30 channels can be unlocked. When more than 30 channels are required for installation, then an ISDN30 is necessary.

With SIP, the number of channels supported is only limited by the bandwidth of the data connection. As SIP is ran on the business's existing WAN, it can be expanded and subtracted to meet the needs of the company. Assuming that the bandwidth of the Internet connection is available to support it, the SIP can split into as many lines as is required. This makes SIP vastly superior to ISDN in terms of scalability.


There are limited backup options offered by ISDN when the BT exchange takes place as it should be possible to divert an external number in a different area. What this means is that other numbers (such as mobile numbers) can redirect to a single number. This allows for you to make a business call from a wireless device while at the same time appearing on the other line as the company number, which allows an enormous amount of flexibility in a business. There are additional redundancy options available for network setup and telephony solution. Should anything in the network malfunction it is important to take the time to check the entire system to ensure everything is installed correctly. Hardware failure is common so if anything is damaged make sure you repair and replace the necessary parts.

It is recommended with SIP trunk solutions that you have a backup ethernet line available. The reason for this is because a physical backup allows you to remain connected at all times. ISDN2 is an emergency option that needs to be considered when SIP malfunctions. Connect the hardware to SIP trunks as well as the ADSL/Fibre connection in the proper places with the router to keep the internet connection in the same building. Keep all redundancies such as firewalls and other connections in place to produce a safe and reliable network and the environment.


ISDN's cost is per channel for every month. One installation fee is required at the beginning of startup and in addition to this a monthly charge for each channel. There are rental fees in the works for DDI number as well as a monthly service for services like CLI.

There is usually a one off of the connection with SIP, and that charges you a month fee with each trunk required. Remain in mind there is a lot of data associated with ISDN, so keep that in mind as you may think that the SIP trunk is cheaper.


Plan running these channels well in advance if you would like to have reliable connection.

When you properly plan everything you can increase the number of ISDN channels you have in total. More telephony hardware may be necessary which requires more installation and lead time.

To add another SIP trunk is a simple and quick process, all you have to do is select however many more channels that are needed for direct connections. It is simple and very easy to scale or reduce to meet the needs of the organisation.

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