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What is a SIP trunk, and when do you need one?

The term sounds technical, but the thing itself is simple: it's the link that connects your existing phone system to the telephone network, over the internet instead of physical lines.

11 September 2026 · 2 min read · The Axivox team

If you already have a phone system, physical or software-based, a SIP trunk is the pipe that brings calls in and sends them out. It replaces old phone lines with an IP connection, without changing your handsets.

What it replaces

In the past, each line was a physical wire, and the number of simultaneous calls was fixed by the number of wires. A SIP trunk carries several calls over a single connection, and capacity is set by a parameter, not by a technician running a cable.

When it makes sense

A trunk suits anyone who wants to keep their phone system and change only the connection to the carrier. That's the case for a business with a recent IPBX, or a site that has invested in its hardware and has no reason to replace it.

When a cloud phone system is simpler

If you don't have a phone system, or if yours is at the end of its life, connecting a trunk to it just means maintaining a machine you might not need at all. A hosted phone system gives you the same numbers with no central hardware to maintain. A trunk is a bridge to what you already have, not an end in itself.

  • You're keeping your IPBX: a SIP trunk is enough
  • Your phone system is at the end of its life: compare it with a hosted phone system
  • You're starting from scratch: a trunk is probably not the right starting point

Either way, your numbers stay yours and can be ported. A trunk is just one way to connect them, and the right choice depends on what you want to keep.

We connect trunks to existing IPBXs just as often as we build full phone systems: the choice depends on what you're keeping.

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