The basics

When you call a business, your call travels over the phone network — a complex infrastructure of wires, exchanges and wireless connections that spans the country. At some point, that call needs to reach the business's phone system.

A SIP trunk is the connection that bridges your hosted PBX in the cloud to the broader phone network. Without it, your PBX could route calls between your own extensions — but it couldn't receive calls from outside or make calls to external numbers.

What does SIP stand for?

SIP stands for Session Initiation Protocol. It's a technical standard — a set of rules that defines how voice calls are set up, managed and ended over the internet. You don't need to understand the protocol itself to use SIP trunking; it's the underlying technology that makes internet-based calling work.

A SIP trunk, then, is a virtual phone line that uses the SIP protocol to carry calls between your PBX and the phone network over the internet — rather than over a traditional copper phone line.

In plain English: a SIP trunk is an internet-based phone line. Instead of a physical cable connecting your office to the phone exchange, it's a secure internet connection that carries your calls to and from the outside world.

How is this different from a traditional phone line?

Traditional phone lines — ISDN, PSTN, or what most people call a "landline" — are physical connections from your office to the phone exchange. Each line carries one call at a time. If you have ten lines, you can handle ten simultaneous calls.

SIP trunks work differently. They're software-defined, which means capacity can be scaled up or down without installing new physical lines. You define how many simultaneous calls you need, and the trunk carrier provisions that capacity in software. Adding more capacity is a configuration change, not a hardware job.

Who provides SIP trunks?

SIP trunks are provided by carriers — companies that operate the infrastructure connecting internet-based PBX systems to the phone network. In Australia, common SIP trunk providers include Vonex, Symbio, MyNetFone, and others.

With a managed hosted PBX like CallPath, you have two options: bring your own SIP trunk (from a carrier you already have a relationship with), or have CallPath manage trunks on your behalf. Either approach works — we configure the connection on your side regardless.

What does this mean for your business in practice?

For most businesses, SIP trunking is invisible. You don't interact with the trunk directly — it's a component that your hosted PBX provider manages as part of the service. You pick up the phone and calls work.

The practical implications are worth understanding, though:

  • Your calls travel over your internet connection — so a reliable, business-grade internet service matters
  • Call quality is generally excellent on a good connection, and often better than older PSTN lines
  • If your internet goes down, inbound calls can automatically divert to a mobile number as a fallback
  • Costs are typically lower than traditional line rentals, particularly for businesses making a high volume of calls

If you'd like to understand how SIP trunking would work specifically for your setup, get in touch and we can walk you through it.