What is a SIP trunk? Your guide to choosing a business SIP trunk provider
Your phone system still runs on ISDN lines. You’ve heard they are being switched off. So what comes next? For most businesses, the answer is a SIP trunk.
It is a virtual phone line. It lets you make and receive calls over your internet connection instead of old copper lines. It connects your phone system to the public phone network, so customers can still reach you on the numbers they already know. Need more lines? You add them in your settings. No engineer visit, no new cabling.
How does a SIP trunk work?
SIP stands for Session Initiation Protocol. It is the technology that sets up, manages and ends a call over the internet.
Here is what happens when a customer calls you:
- The customer dials your business number from a mobile or landline.
- The call travels through the public phone network to your SIP trunk provider.
- Your provider sends the call over the internet to your phone system.
- Your phone system rings the right person, team or call queue.
Outgoing calls follow the same route in reverse.
Each trunk carries a number of channels. One channel equals one call at the same time. So if your team of 12 never has more than 8 calls going at once, you need around 8 channels. Not 12, and certainly not the 30 that came in an old ISDN30 bundle.
Wondering how this differs from a single SIP line? Read our guide on SIP lines.
SIP trunk vs PRI: what’s the difference?
PRI (Primary Rate Interface) is the business version of ISDN. In the UK you probably know it as ISDN30. It uses physical copper lines and comes in fixed bundles of channels. It has served businesses well for decades, but it was never built for the way you work today.
Here is how the two compare:
| PRI / ISDN30 | SIP trunk | |
| Connection | Physical copper lines | Your internet connection |
| Channels | Fixed bundles, often with a minimum | Add or remove one channel at a time |
| Adding capacity | New line installation, engineer visit | A change in your settings |
| Location | Tied to one building | Numbers work from any location |
| Future | Being switched off | The standard going forward |
The biggest difference? Flexibility. With PRI, your phone lines live in one building. With SIP, your numbers move with you.
Why upgrade now?
Openreach is retiring the UK’s PSTN and ISDN networks, and the final switch off is set for 31 January 2027. New ISDN and analogue lines are already no longer on sale. If your business still relies on them, now is the time to plan your move. Read more in our guide on what ISDN is and what comes next.
Making the move before the deadline means you choose the timing. You test your setup, train your team and avoid a last minute rush.
The benefits of SIP trunks
Switching changes more than where your calls travel. Here is what you notice day to day:
You grow without the hassle. Opening a new office or hiring a seasonal team? You add channels when you need them and remove them when you don’t.
You keep your numbers. In most cases you can port your existing numbers when you switch. Your customers won’t notice a thing.
You work from anywhere. Your business number is no longer tied to a desk. A colleague at home or on the road makes calls from the same number as the office.
You stay reachable. If your office loses its connection, you can route calls to another location or to mobiles. Customers still get through.
You see what you pay for. You pay for the channels you use, not for copper lines that sit idle most of the day.
Using a SIP trunk with Xelion
The trunk brings the call in. What happens next depends on your phone system. That is where Xelion comes in.
Xelion is a cloud phone system that works with SIP trunks. Picture this. A customer calls your main number. The call comes in over the internet. Xelion sends it to the right hunt group or call queue. Your colleague sees who is calling and what was discussed last time, before they even pick up.
With Xelion you get:
- Clear call routing with auto attendant, call queues and hunt groups
- An overview of who is available, so you transfer calls faster
- Call recording and reports to see how your team handles calls
- Apps for desktop, mobile and browser, plus support for desk phones
- A link with Microsoft Teams, so your team can call from the tool they already use
Everything is included in one licence. No separate modules to add later. Xelion is built in house and is GDPR compliant, and more than 20,000 businesses use it every day.
How to choose a business SIP trunk provider
Not every business SIP trunk provider offers the same service. Before you sign, ask these questions:
- Can you port all my existing numbers?
- What happens to my calls if my internet connection drops?
- How many channels do I need, and how quickly can I add more?
- Does the SIP trunk work with my current phone system?
- Who do I call when something goes wrong, and how fast do they respond?
A good business SIP trunk provider answers these clearly and helps you plan the switch step by step.
