What is SIP Trunking?

With the ISDN switch off creeping up, we thought it would be helpful to put a short guide together on one of the options for businesses switching to IP telephony over the next few years – SIP Trunking. 

We look at: 

  • What is SIP Trunking?
  • Trunks vs. Channels 
  • Who should consider SIP Trunking 
  • Features & Benefits 
  • Key things to consider when switching 

Let’s start off by looking at what SIP Trunks actually are. 


What is SIP Trunking?

SIP stands for Session Initiation Protocol and is a way to make and receive calls over the internet. A ‘trunk’ refers to a collection of phone lines shared between users.  

Designed to replace traditional phone lines, a SIP trunk is a virtual link between your PBX and PSTN using a broadband internet connection and uses virtual phone lines rather than physical wires. SIP trunking is the service that allows you to use the SIP protocol on a public network. 

Whilst legacy PBX systems used ISDN to connect calls via the PSTN, a modern IP PBX also needs a way to connect their exchange to the PSTN with SIP trunks becoming the industry standard over recent years. Especially with the ISDN being phased out over time. 


Trunks & Channels – The Terminology 

With words like trunks and channels thrown about, it’s not always obvious what they mean. Let us explain. 

A SIP Trunk is essentially a collection of channels, each one able to handle a set number of simultaneous calls with the number of channels you need essentially boiling down to the number of simultaneous calls that you’ll be making as a business. 

Each channel allows two concurrent calls: one incoming or outgoing. So, you may need more than one channel if you know that you’re going to be regularly having more than one or two concurrent calls either outgoing or incoming. 

If you do go ahead with SIP trunking you need to understand from your provider, the number of channels (concurrent calls) you get within each trunk to help you understand how many trunks you need to get the right amount of channels. 


Who should consider SIP Trunking? 

If you’re one of the businesses in the UK still with a legacy PBX system using ISDN lines then you should be strongly considering switching to SIP Trunking. 

With BT gradually phasing out its PTSN and ISDN networks, businesses need to make sure they switch over to IP telephony services sooner rather than later. 

If you have an IP compatible PBX then it’s simple to get SIP trunking setup and working and we’d recommend that as one of the solutions to ensure you’re not impacted by the switch off. 

If you’re considering switching telephony solutions in light of the 2025 switch-off we’ve written a blog on this here. 


Why are businesses switching to SIP Trunking? 

Whilst it seems that SIP Trunking is the same as any other telephony solution in that it enables businesses to make and receive calls, there are lots of great benefits that businesses can take advantage alongside futureproofing your telephony system. 


Depending on your usage, trunks are easy to scale up and down. Unlike ISDN lines where they have to be physically installed, if you need to add or remove trunks or increase capacity, this can be done easily and simply. 

Cost savings 

SIP trunks allow businesses to more accurately pay for what they use when compared with traditional systems which are usually overquoted in terms of capacity. With lower line rentals and call costs as well SIP trunks provide savings across the board, especially if you have high outbound call volumes. 


As SIP trunks aren’t location restricted you can use any numbers you choose regardless of geography. This means you can also move office or location without having to change numbers. As it’s also a virtual installation rather than a physical one you can get setup in hours rather than days. 


What do I need to consider? 

If you’re considering switching to SIP Trunking here are a few things we suggest to consider and bear in mind: 

How many channels will you need? 

As touched on earlier, make sure that you have the right number of channels quoted for – for example if you aren’t likely to have more than 4 concurrent calls (either making or receiving) at any one time make sure that you have 4/5 channels within your SIP Trunk. 

Do you need any existing numbers porting over? 

If you have existing numbers that you want to keep, make sure that this is communicated to your provider in good time as these can sometimes take a few weeks to port.  

Do you need any new numbers? 

If you require new numbers when switching provider make sure this is also communicated in advance to ensure that new numbers are added to your service on install. 

How many minutes do you require? 

Make sure that with any provider you go with to ensure that you have the minutes that you require as part of the bundle, or if they aren’t included are aware of the call charges that will occur should you use that specific system to avoid unexpected bills! 


Need some extra help? 

The switch-off does provide a challenge for businesses who haven’t been considering moving their phone system to another technology so it’s completely normal to feel a bit overwhelmed or unsure of the best solution for your business. 

At Connexin we’ve been supporting business making the switch from legacy telephony to VoIP systems since 2006 so have helped countless businesses like your own put in solutions that fit for their teams. 

If you have any questions or queries our team of specialists are on hand to support you in making the best decision for you, no matter what that looks like. If you’d like some support you can chat to our team or book in a phone system audit with one our specialists using the form below where we’ll chat through your current requirements and work alongside you to put in place the best system for your business.