SIP vs ISDN – What are the advantages?

The Corbel Blog

When businesses are upgrading their communication systems one of the most effective system upgrades is the move to using SIP Trunking. SIP Trunking offers a number of advantages over more traditional ISDN lines, and not only in terms of costs. BT has also announced that they intend to switch off their ISDN network by 2025 so all customers will have to move to a SIP Trunk-based service.


What is SIP Trunking?

Basically, a SIP Trunk is a virtual IP-based service that delivers telephone lines using an external data connection as the bearer for the purpose of making and receiving calls. SIP Trunking has been designed to make voice and video communication easier and more efficient than the regular method.



Advantages of SIP Trunking

SIP has been around for about 15 years, making it a relatively new advancement in the communication system and it has proved to provide a number of advantages to businesses. The main advantages to using SIP over ISDN include:


Consolidated voice and data network

Using a single network for both your voice and data reduces complexity, management and costs while making your system more efficient than alternative voice and video systems. The single network will also increase voice functionality, bringing a full range of facilities to your business communications including instant messaging, voice calls, video conferencing and the ability to synchronise address books between different devices. You’ll also have the ability to record conference calls for staff that cannot be present, or to archive for future reference or training purposes.


Reduced call costs

SIP trunking helps to cut down on call costs and you can make significant savings depending upon your usage and call plan selected. SIP also helps to eliminate costly PRI lines and this will help you make large savings over time. Your business can also benefit from free internal calls across sites on your network.


Valuable ROI

Your business can benefit from a decent ROI iwhen moving to SIP Trunking due to not paying for unecessary services and equipment. Another saving is gained because your voice traffic is part of your existing data bandwidth so you’re only paying for one bandwidth allocation rather than one for data and one for voice. Overall, your telephony bills will be reduced and your call quality will be increased, guaranteeing you of a respectable ROI.


Efficient failover and effective disaster recovery

In the event of a distaster or a failure at your premises, SIP Trunks allow voice traffic to be re-routed to a different location, backup-site or telephone. This can be done quickly and easily, making it an important part of your disaster recovery plan.


High volume traffic? No problem

If you have a high volume of voice traffic, you can benefit from facilities such as load-balancing and multiple outbound routes which can ensure whatever the volume of traffic you experience, the network will handle it. No more need for complex multiple point to point lines and separate backup circuits.


Remote working and no more missed calls

SIP Trunking allows you to route calls automatically to a mobile phone, making remote working much easier and means your business will suffer less from missed calls when employees are on the move. Calls can be routed to a mobile if the office line is engaged, allowing you to receive calls at any time. Your SIP Trunks can be configured to push the call to the next available line, divert it to another number or route it to a mobile phone.


Numbers and porting numbers

SIP isn’t geographically dependent so if you move premises you can keep the same number, even if you move across the country. UK numbers can easily be provisioned and ported.


Rapid deployment

SIP is quick and simple to setup compared to ISDN. Installation of an ISDN line is typically around six weeks, whereas a SIP Trunk can be installed in as little as 48 hours.



SIP Trunking allows you to grow or shrink the number of extensions, something that’s not possible when dealing with ISDN. As SIP isn’t using physical lines you can scale it according to your business needs and plan for the peak times in a more professional way. Plus, with SIP you only pay for what capacity you’ve used, even when you scale your extensions up and down.


Common argument against using SIP

There are some common arguments against using SIP instead of ISDN but these can be easily solved, including:

“Our PBX doesn’t support SIP”
This is probably one of the most popular responses. This situation is also often easily resolved by utilising a low-cost IP gateway platform.

Perhaps you’re in a contract already
Call charges via SIP Trunks are often significantly lower than those charged using traditional services such as ISDN. Such long-term savings may well outweigh any early termination fees associated with an existing contract.

“We don’t have the data line to support SIP”
If you have an existing connection to the internet; SIP gas the ability to get you up and running quickly with technology to provide guaranteed quality of service.

In summary, SIP trunking is a reliable approach that makes voice communication swift and efficient. Even with its advancement, the protocol is cheaper than the regular voice communication system, partially due to eliminating costly multiple networks and using a single manageable network. SIP also enhances business productivity by improving collaboration within your workforce. Put simply, SIP is the best option to choose for the communication system in any business.

Looking for further information on SIP Trunking technology? Visit our SIP and SIP Trunking Solutions page for further information and videos.

Interested in finding out how much migrating from ISDN to SIP could save your business? Give us our BT solutions team a call on 01473 241515 or use our contact form.