What is SIP trunking?

SIP trunking, defined

SIP trunking is the technology that connects a business phone system to the public phone network over the internet. Rather than running physical lines into your building to carry calls, you connect your phone system to a SIP trunk provider, and your calls travel as data across your broadband to reach the outside world. It is the modern replacement for the old ISDN lines that businesses used for decades.

The name packs two ideas together. SIP is the protocol that handles the call. A trunk is the line that carries the calls. Once you understand both halves, the whole thing falls into place, so it is worth taking each in turn.

What SIP actually is

SIP stands for Session Initiation Protocol. It is the signalling language that sets up a call, rings the other end, connects the two parties and then ends the call when someone hangs up. SIP does not carry the audio itself; it is the part that says "ring this number, connect these two ends, now finish". Think of it as the postal address and the knock on the door rather than the conversation.

SIP is one of the building blocks of VoIP, the broad idea of carrying voice calls over the internet. That is the key relationship: SIP trunking is a specific use of VoIP. All SIP trunking is VoIP, but plenty of VoIP, such as a fully hosted phone system, never involves you buying a trunk at all.

What a trunk is

In old telephone language, a trunk is the line that carries calls between a phone system and the wider network. A traditional ISDN trunk carried a fixed number of calls, one per channel, down physical lines that an engineer had to install. If you wanted more simultaneous calls, you ordered more lines.

A SIP trunk does the same job, but as data over a single internet connection. Instead of one call per copper channel, the trunk carries many concurrent calls as packets across your broadband. You scale the number of simultaneous calls up or down by changing your plan, not by sending an engineer to lay new lines. That flexibility, plus the lack of line rental for each channel, is why SIP trunks have largely replaced ISDN for businesses that run their own phone system.

How SIP trunking works, in plain terms

The chain is simpler than it sounds. Your phone system, the PBX, holds your extensions, menus and routing. To reach anyone outside the business, it needs a path to the public phone network. A SIP trunk is that path.

1. Your PBX places or receives a call. Someone dials out, or an external caller dials your number, and the call needs to cross from your phone system to the outside world.
2. SIP sets up the call. The SIP protocol negotiates with the trunk provider to ring the right number and connect the two ends.
3. The call travels over the SIP trunk. The audio crosses your internet connection to the provider as data packets, sharing one trunk across many calls at once.
4. The provider connects to the PSTN. The trunk provider hands the call onto the public phone network so it can reach any landline or mobile, and brings inbound calls back the same way.

The caller on the other end notices nothing. They dial a normal number and have a normal conversation. What has changed is that the line into your phone system is now an internet connection rather than a bundle of physical wires.

SIP trunking vs a cloud phone system

This is the part that trips people up, so it is worth being precise. SIP trunking modernises the lines into a phone system, but it assumes you still run that phone system yourself. A complete cloud phone system goes a step further and gives you the phone system and the lines together, as one managed service, with nothing for you to run.

Both are valid. If you have invested in a PBX and want to keep it, a SIP trunk is a sensible way to retire your ISDN lines without replacing the whole system. If you would rather not run a phone system at all, a cloud phone system removes that job entirely.

Where Voxora fits, honestly

It would be easy to muddy this, so here is the straight version. Voxora is a complete cloud phone system: the phone system, your numbers and your calls delivered as one service. There is no separate PBX for you to run and, for that reason, no separate SIP trunk for you to buy and wire in. We are the all-in-one alternative to running your own PBX plus a trunk, not a standalone trunk supplier.

So for most small and medium businesses, the question of which SIP trunk to choose simply does not arise. You sign up, you get your numbers and your call handling in one place, and the lines are part of the deal. SIP trunking remains the right tool for a business that has good reasons to keep its own PBX and just wants to modernise the lines into it. If that is not you, the simpler route is to let the whole thing be managed as cloud communications and skip the trunk decision altogether. For a closer look at the managed-service model, our guide on what cloud telephony is sets it out in full.

Why this matters before 2027

There is a deadline driving all of this. The old PSTN and ISDN networks are being switched off on 31 January 2027, so every UK business still on physical lines has to move to an internet-based service before then. SIP trunking is one of the answers: a business keeping its own PBX can swap ISDN for a SIP trunk and carry on. A cloud phone system is the other answer, replacing both the lines and the PBX in a single move.

The point is that doing nothing is not an option, and the two routes solve the same deadline in different ways. We cover the timeline and what to do about it in our guide to the 2027 PSTN switch-off. Whichever route suits you, the sooner you plan it, the smoother the change.