In considering switching your business from traditional PSTN telephone systems to VoIP-based technology, you won’t go very far before you encounter SIP trunking. But what exactly is it and what does it do?
In simple terms what SIP (Session Initiation Protocol) trunking does is to connect your premises to the phone network. But unlike the older system where you used an ISDN line, SIP trunking does the job via an internet connection.

This confers a number of benefits including greater flexibility, lower cost and improved reliability. It’s not surprising then that SIP trunking has grown significantly in popularity in recent years. Let’s have a look at some of the advantages in more detail.

Cheaper voice termination

One of the biggest advantages of VoIP systems is cheaper calling, because traffic is routed more efficiently via the internet it can always find the lowest cost route to its destination. SIP trunking also allows you to make savings because it costs less in rental than an ISDN line. Not only that, but it can often make it possible to make landline calls free of charge – certainly within the UK – and other types of calls may be cheaper too.
Strategic considerations

Many businesses are now moving to unified communications (UC) strategies, bringing together all of their comms – voice, email, instant messaging, video conferencing – in one easy to manage strategy. SIP trunking can help here as it can allow users access to a unique number whichever network they are connected to. It also allows greater flexibility in connecting devices. If you are moving to the cloud, something which often goes alongside a UC strategy, then SIP trunking can help you there too.


In many cases, when businesses upgrade their communications they take the opportunity to move their PABX to the cloud. SIP trunking is usually compatible with this, but if you want to retain your old in-house PABX, you need to ensure that it’s going to work with the new SIP channel. Many providers will carry out interoperability testing to help you ensure that your equipment will continue to work smoothly after you switch.

If you have multiple sites, SIP is of value here too. It’s possible you have existing ISDN connections between sites; these are inflexible – limiting the number of lines available – but also have relatively high rental costs. Switching to SIP means that you can reduce the number of connections and also the cost. It also means that you don’t need to maintain a PABX at each site, you can centralise the operation in one place or in the cloud.

Flexibility and scalability

One of the major drawbacks of older systems is that ISDN lines limit the number of lines you can have on each connection. Expanding that means renting another line.

SIP trunking offers you far more flexibility. If you are a business that experiences seasonal peaks, you no longer need to pay for extra capacity that will lie unused for most of the time. You can quickly and easily add or remove trunks as needed to cope with demand.

There’s also flexibility in numbering when using SIP. This means that you are free to move premises and keep your old numbering, even if your new base is in a different exchange area. You can use this to your advantage if you operate in different parts of the country too, ensuring that your customers have a local number to call, even if that call is actually answered centrally elsewhere.


Inevitably with any system that uses the internet, there are going to be concerns as to security. Some firms worry about criminals being able to steal call time or intercept confidential information.

While these concerns are understandable, if a SIP trunking system is properly configured and protected, then it should be no riskier than a conventional phone system. Ideally, your SIP traffic should have a dedicated router and firewall to keep it separate from the rest of your internet traffic. Your service provider can also help you to ensure that it’s configured correctly to meet your needs and that the traffic is managed so that it won’t interfere with other services.

There are advantages when it comes to overall business resilience too. As part of a business recovery plan, you need to ensure that you can keep your communication up and running. SIP trunking can allow you to quickly divert lines to another location, meaning minimal disruption to your business in the event of a failure.

Businesses rely on communication and SIP trunking offers many benefits in terms of ensuring that your voice services run smoothly, can cope with demand and can keep going even if you experience problems. Add in the potential savings and it’s a technology you can’t afford to ignore.