<img height="1" width="1" src="https://www.facebook.com/tr?id=899063296883775&amp;ev=PageView &amp;noscript=1">

Blog Articles

SIP Trunking vs ISDN

[fa icon="calendar"] 27/4/2015 / by Toni Mulvaney

Traditionally businesses have used fixed line solutions such as analogue or digital telephone lines for making and receiving calls. SIP trunking is a virtual line that replaces these traditional methods, allowing your business to connect voice calls over an internet connection instead. So what is SIP and what is ISDN - how are they different?


With ISDN the connections are physical: The circuit is brought into your building from the BT exchange by a BT engineer and a small box or a number of small boxes are installed within your building for your business phone system to physically connect to. Each ISDN30 circuit (30 trunks/channels) is one box and each ISDN2 circuit (2 trunks/channels) is a box. For example 8 channels of ISDN2 would mean 4 small boxes being installed. 

5 Problems with SIP Trunks - Read the Blog
With SIP trunking the connections are virtual: The number of trunks you can have is almost up to you. There may be restrictions due to available bandwidth and data connection speeds but there is no physical termination hardware required within the building.


Scaling up ISDN requires the installation of new circuits and additional ISDN boxes. With ISDN2 you can go up in increments of 2 channels/trunks at a time. With ISDN30 you begin with a 30 channel pipe being installed into the premises and you can 'unlock' anything from 8 channels, as a minimum, up to the 30 channels depending on your requirements. You can also have more than 1 ISDN30 pipe installed if you require more than 30 channels.

With SIP you can scale up or down easily and quickly - it’s a simple request and change to increase or decrease the number of SIP trunks you have. 



ISDN offers limited backup options if the BT exchange were to go down, they should be able to put on a divert to another external number elsewhere, i.e. another site or a user's mobile number however this often means that all of your numbers (main numbers and DDI's) can only divert to a single number as a divert which is often not ideal. Other options may be available for redundancy depending on your telephony solution and network setup. An ISDN failure could also be due to the telephony equipment itself so it is important to make sure you have a suitable maintenance support agreement in place to repair/replace any parts in the event of hardware failure. 
With a pure SIP trunk solution there are several things to factor in when discussing backup. We would recommend that you always look to have a physical connection, i.e. ISD5 Benefits of SIP Trunks - Read the BlogN2 as an emergency option should your SIP fail. For a SIP solution you require several pieces of hardware and connection, the SIP trunks, the ADSL/Fibre connection they sit on, the Router that brings the internet connection into the building, internal firewalls, plus any hardware that convert the SIP to connect it to the telephone system. It can therefore be costly and often not viable to have redundancy in place for all of these possible scenarios.


With ISDN the cost is per channel per month. There is an initial installation charge followed by ongoing monthly rental per channel. There are also planning, connection and rental fee's for DDI numbers and also monthly select service fee's for services such as CLI. 
With SIP there is usually a one off connection fee and then a monthly fee for each trunk required. The rental for a single SIP trunk is predominantly cheaper than that of ISDN however because you will need to factor in a high level of data connection, i.e. ADSL or fibre to run the SIP service, the overall cost can be much higher and more comparible to that of ISDN.


If you wish to increase the number of channels you have with ISDN then often you need to plan for this well in advance as considerable BT lead times can often play a big part in how soon an increase will take effect. You may also need additional telephony hardware which again may incur a lead time for installation.
Adding additional SIP trunks is a quick and simple process. You just select how many more channels are required and its almost instantaneous.


Topics: Business Telephone Systems

Toni Mulvaney

Written by Toni Mulvaney

New Call-to-action
10 Mistakes to Avoid When Buying a Business Phone System

Subscribe to our Blog

8x8 Voip Phone System
Guide to Buying a Telephone System