What Is a SIP Trunk, in Plain English

Forget the acronyms for a second. Imagine the old copper phone lines running into your building. Each physical line could carry one call at a time, so a busy office needed a bundle of them. A SIP trunk does the same job, except there is no copper and no ISDN cable. It is a virtual set of phone lines delivered over your existing internet connection.

SIP stands for Session Initiation Protocol, which is simply the digital language that sets up, manages and ends a phone call across the internet. The word "trunk" is borrowed from the old telephone world, where a trunk was the group of shared lines connecting a business to the wider network. Put them together and a SIP trunk is a modern, internet-delivered replacement for those physical trunk lines.

Here is the part that saves money. A single SIP trunk can carry many simultaneous calls at once, and each of those simultaneous calls is called a channel. Instead of paying for one line per handset, you buy the number of channels that matches how many people are actually on the phone at the same time. Your phone numbers, including your main business number, ride along on that trunk and can be ported across without changing.

SIP vs VoIP: Clearing Up the Confusion

People use "SIP" and "VoIP" as if they mean the same thing, and that causes plenty of muddled quotes. They are related but not identical.

• VoIP (Voice over Internet Protocol) is the broad concept: carrying voice calls over the internet rather than the traditional telephone network. It is an umbrella term.
• SIP is the specific signalling protocol that most business VoIP relies on to start, control and end each call. It is the mechanism under the bonnet.

In everyday terms, a SIP trunk is the business-grade way of delivering VoIP lines to a phone system. So almost every SIP trunk is VoIP, but not every VoIP setup uses a traditional SIP trunk. A cloud phone app on your mobile, for example, is VoIP too, but you are not managing a trunk yourself. If you want a deeper look at the fundamentals, our business VoIP guide for Australia unpacks it further.

The ISDN Switch-Off and Why You Must Migrate

If your business is still running phones over ISDN or PRI, this section matters most. Telstra fully decommissioned its ISDN network by mid-2022. That was not a temporary outage or a pricing nudge, it was a permanent shutdown of the underlying technology. Any service still clinging to ISDN is living on borrowed time, often on interim arrangements that carry their own risks and costs.

SIP is the recognised replacement. Migrating means porting your existing numbers onto a SIP trunk (or onto a hosted cloud phone system) so that everything keeps ringing exactly as before, just delivered over the internet instead of a legacy circuit. Customers dialling your number notice nothing. Behind the scenes, you swap an ageing, expensive technology for a flexible one.

The financial upside is real. ISDN channel rental commonly ran to around $30 to $60 per channel each month before you even made a call, so businesses moving to SIP frequently trim their monthly voice spend by roughly 30 to 60 percent. Those figures are illustrative and depend on your channel count and call volumes, but the direction of travel is consistently downward.

What SIP Trunking Actually Costs in 2026

SIP pricing looks very different from an ISDN bill, and usually much friendlier. Rather than paying steep rental on each physical line, a SIP trunk cost is normally built from a few simple parts:

• Per-channel charge — a modest monthly fee for each simultaneous call your business can make. This is where the biggest saving against ISDN shows up.
• Call usage or bundles — either pay-as-you-go call rates or an included bundle. Many Australian providers now include generous or unlimited calls to local, national and mobile numbers.
• Number rental — a small charge for the phone numbers you hold, including any inbound 1300 or 1800 services.
• One-off porting — moving your existing numbers across, which reputable providers keep straightforward. Uniden Voice Over Cloud offers free porting for a limited time.

Because the per-channel cost is so much lower than ISDN line rental, the total monthly figure usually drops significantly. Treat any headline price as a starting point and insist on a written quote against your real call patterns. For a fuller breakdown of what an entire phone setup costs, including handsets and licensing, see our business phone system cost guide for Australia.

Sizing Channels and Bandwidth

The most common mistake businesses make is buying one channel per person. You almost never need that many. Plan channels around concurrent calls, not headcount.

Think about how many staff are genuinely on the phone at the same moment during your busiest hour. A 30-person office where only eight people are ever talking at once may need just eight to ten channels, with a little headroom for peaks. Buying 30 channels for that office would be pure waste.

Bandwidth is the other half of the equation. As a rough capacity rule, each concurrent call uses around 100 Kbps in each direction. Multiply your channel count by that figure, leave comfortable overhead for everything else on your connection, and confirm your internet can handle it. A wired, business-grade connection with quality-of-service settings gives the most reliable call quality. If you ever run into crackle, dropouts or one-way audio, our guide to troubleshooting VoIP and SIP problems in Australia is a handy reference.

These figures are illustrative starting points. Voice compression, call recording and other traffic on the same link all shift the numbers, so treat the table as a planning guide rather than a guarantee.

SIP Trunk vs Hosted Cloud PBX: How to Choose

This is the decision that trips up most Australian businesses, so let us make it simple. Both options give you modern internet calling. The difference is who owns and runs the phone system.

So how do you pick? A quick way to decide:

• Choose a SIP trunk if you already have a capable PBX you are happy with, your IT team likes managing it, and you mainly want to modernise the lines feeding it and cut ISDN costs.
• Choose a hosted cloud PBX if your hardware is ageing, you would rather not maintain a system at all, or you want built-in features like AI call agents, a mobile app, business SMS and unified messaging without stitching them together yourself.

There is a middle path many businesses never hear about. You do not have to choose blindly today. If you are weighing up whether to keep managing hardware at all, our hosted PBX vs on-premise PBX comparison for Australia lays out the trade-offs in detail. And if you are running 3CX right now, moving to a fully managed platform can be smoother than it sounds — our 3CX to Uniden Voice migration guide walks through the whole process.

Worth knowing: with Uniden Voice Over Cloud, a hosted cloud phone system does not mean paying extra for the clever bits. AI call agents are included as standard, alongside more than 50 features, with no minimum number of users. So the "replace the PBX" option often ends up doing far more than a SIP trunk into legacy hardware ever could.

Compliance: 000, Numbering and Data Sovereignty

Moving to SIP is not just a technical exercise, it comes with Australian obligations that a good provider handles for you. Three matter most.

Triple Zero (000) must keep working. Australian providers are required to meet Triple Zero and IPND obligations, which means emergency calls and the correct registered address details continue to function after migration. Always confirm your 000 configuration and service address as part of the cutover, and test it before the old service is switched off.

Numbering rules apply. Providers must comply with the Telecommunications Numbering Plan, which governs how numbers are allocated and ported. This is what lets you keep your existing numbers legitimately when you migrate.

Data sovereignty is a genuine differentiator. Where your call data and recordings are stored is not a small detail under the Privacy Act 1988 and the Australian Privacy Principles. Uniden Voice Over Cloud is 100 percent Australian-hosted, so your data stays onshore, backed by 24-hour local support and a dedicated account manager. That is a meaningful contrast with providers who route support and storage offshore.

Your Step-by-Step Migration Checklist

Migrating off ISDN or onto SIP is far less daunting with a clear plan. Here is the sequence that keeps cutovers smooth.

• 1. Audit your numbers and channels. List every phone number you use and count your true concurrent-call peak, so you buy the right number of channels.
• 2. Check your internet. Confirm your connection has the headroom for your channels at around 100 Kbps each, and ideally enable quality-of-service for voice.
• 3. Decide SIP trunk or hosted cloud PBX. Use the comparison above, and factor in the age of your current hardware and the features you actually want.
• 4. Get a written, all-in quote. Make sure channels, call bundles, number rental, porting, 000 and support are all spelled out.
• 5. Port your numbers. Your provider manages the port so your numbers move without interruption. Uniden Voice Over Cloud offers free porting for a limited time.
• 6. Test, then cut over. Verify inbound and outbound calls, voicemail and especially Triple Zero before the old ISDN service is decommissioned.

Done properly, the switch is invisible to your customers and a relief to your finance team. If you would like the wider picture of how these systems fit together, our business VoIP guide is a good companion read.

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