Telecoms, like any other area of technology, is somewhat laden with jargon. If you’re looking at VoIP (Voice over IP) technology for your business call needs, you’ve doubtless come across the terms SIP (Session Initiation Protocol) or SIP trunking and PRI (Primary Rate Interface). But what do these Three Letter Acronyms (TLA) mean exactly and why are they important?


The traditional way of providing business telecoms is via a PABX system that is linked to the public telephone network (PSTN) via a digital service line (ISDN). The PRI is divided into channels each of which is effectively dedicated to a phone line.

The advantage of this is that it delivers a high-quality connection. The disadvantage is that ISDN lines are relatively expensive to rent and present a bottleneck should you need to expand your capacity.

SIP trunking and wholesale VoIP termination rates

The modern alternative is SIP trunking. Like PRI, this connects your internal PABX to the PSTN, but it does so using internet protocol (IP) technology. This means that data is divided up into packets that are routed over the internet, so there are no dedicated channels as with PRI. The only limit here is the bandwidth available on the internet connection, so it’s easier to scale up to provide more calling capacity should you need to do so.

Because a SIP trunk is a series of virtual connections, there is no need for physical termination of each link as there is with PRI. This means less hardware is needed – with consequent extra cost savings. Many recent PABX systems can work with either technology, so there may well be minimal need for hardware changes. If you’re setting up a completely new system then there’s also the option of a hosted PABX in the cloud, which could mean that there’s no requirement for on-site hardware at all.

Quality of service (QoS) is an area that many people worry about when switching to IP based systems. Call quality with ISDN systems is high as you effectively have a dedicated channel for each call. As we said above, the key here for SIP systems is bandwidth, it’s therefore vital to have a fast, reliable internet connection in order to avoid the packet delays that would lead to a reduction in call quality.


The main reason firms seek to switch to IP-based systems is usually cost. By switching to a SIP trunked system, you are of course saving on the rental costs of a dedicated line which can be several hundred pounds per month. But there are also savings to be made on call costs. These vary between providers, but offer landline calls free of charge and cheap rates for calling mobile and international numbers which makes IDT the number 1 choice.

You can save on your business mobile costs too, by using apps that allow mobile users to access the SIP  system and make calls via it rather than through the mobile network.