Table of contents
- What is SIP trunking?
- How does SIP trunking benefit your business?
- Is SIP trunking better than a full cloud PBX switch?
- Setting up SIP trunking — what’s involved
- What to look for in a SIP trunking provider
- FAQ
If your business has a phone system you’re genuinely happy with, hardware you’ve already paid for, extensions staff know how to use, the idea of ripping it all out just to get off an ageing copper line probably doesn’t sit well. The good news is that in most cases, you don’t have to.
SIP trunking lets you keep your existing PBX hardware and simply change how calls are delivered to it, over the internet instead of a physical copper line. For a lot of Australian small businesses navigating the NBN switch-off, it’s the option that solves the urgent problem, an ageing copper connection, without triggering the bigger, more disruptive decision of replacing a phone system that’s otherwise working fine.
This guide explains what SIP trunking actually is, how it differs from a full move to cloud PBX, and what to check before signing up with a provider.
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What is SIP trunking?
SIP trunking (Session Initiation Protocol trunking) is a way of delivering phone calls to your existing PBX system over the internet, rather than through traditional copper phone lines. Instead of your PBX connecting to the public phone network through physical lines, it connects through a “virtual” trunk supplied by your provider over your internet connection.
In practical terms, this means your business keeps its current phone hardware, handsets, PBX box, extensions, and everything staff are used to, while the underlying connection method changes. It’s a middle ground between a full traditional phone line and a complete cloud-based overhaul.
How does SIP trunking benefit your business?
Lower call costs. Routing calls over the internet instead of the traditional telephone network typically reduces per-call costs, particularly for long-distance and interstate calls.
No hardware replacement. If your PBX supports SIP, and most modern systems do, you avoid the cost and disruption of a full hardware swap.
Flexible capacity. SIP trunks can usually be scaled up or down more easily than physical phone lines, useful if your call volume fluctuates seasonally or you’re adding staff.
A path off copper. As Australia’s copper network is progressively decommissioned, SIP trunking gives businesses a way to move off it without a bigger system change happening at the same time.
Simplified management. Multiple phone lines can often be consolidated into a single SIP trunk, reducing the number of separate services your business needs to manage.
Is SIP trunking better than a full cloud PBX switch?
| SIP trunking | Full cloud PBX | |
|---|---|---|
| Existing hardware | Keeps your current PBX and handsets | Typically requires new cloud-compatible hardware or software |
| Setup disruption | Lower, connects to what you already have | Higher, involves a full system migration |
| Best suited to | Businesses happy with their current PBX, wanting to move off copper | Businesses wanting modern features (mobile apps, call analytics, remote flexibility) |
| Feature set | Limited to what your existing PBX supports | Broader, cloud-native features usually included |
| Long-term scalability | Good, but capped by your PBX’s own capabilities | Very high, scales with the provider’s platform |
| Cost profile | Lower upfront cost, ongoing per-trunk pricing | Per-user monthly pricing, replaces hardware maintenance costs |
Neither option is universally “better,” it depends on where your business is starting from. If your current PBX is relatively modern and staff are comfortable with it, SIP trunking solves the copper problem with minimal disruption. If your hardware is old, limited, or already causing headaches, a full cloud PBX migration may deliver more value for a similar level of change.
Read more: Hosted PBX Explained: Skip the Server Room →
Setting up SIP trunking — what’s involved
- PBX compatibility check – your provider confirms your existing phone system supports SIP, most systems installed in the last decade do.
- Internet connection assessment – since calls now travel over your internet connection, your provider checks your bandwidth and reliability can support your expected call volume.
- Trunk provisioning – your provider sets up the SIP trunk and configures it to connect with your existing PBX.
- Number porting – your existing business number is transferred across so nothing changes from a customer’s perspective.
- Testing before cutover – calls are tested on the new trunk before your old line is switched off, avoiding any gap in service.
- Go live – calls start routing over the new SIP trunk on an agreed date, with your old copper line decommissioned once everything’s confirmed working.
What to look for in a SIP trunking provider
1. Confirmed PBX compatibility. Get written confirmation your specific PBX model and version supports the provider’s SIP trunk before committing.
2. Clear, per-trunk pricing. Understand exactly what’s charged per trunk, per call, and whether there are any bundled minutes included.
3. A backup plan for internet outages. Since calls now depend on your internet connection, ask what happens, and whether calls can fail over to a mobile, if your connection drops.
4. Managed number porting. Confirm your existing number transfers without any gap in service, and check the timeline before disconnection.
5. Local, contactable support. If a trunk needs troubleshooting, Australian-based support that understands your specific PBX setup matters more than a generic support queue.
FAQ
Do I need to replace my phone system to use SIP trunking?
Usually not. Most modern PBX systems support SIP trunking, meaning you can keep your existing hardware and just change how calls are delivered to it.
Is SIP trunking cheaper than a traditional phone line?
Generally, yes, particularly for interstate and long-distance calls, since call traffic travels over the internet rather than the traditional telephone network. Exact savings depend on your call volume and current plan.
Will SIP trunking help with the NBN switch-off?
Yes. SIP trunking moves your call delivery off the copper network and onto your internet connection, which addresses the switch-off directly without requiring a full phone system replacement.
Can I keep my existing business number with SIP trunking?
Yes. Your number is ported across as part of the setup process, so customers keep dialling the same number they always have.
What happens if my internet goes down?
Since SIP trunking relies on your internet connection, an outage will affect calls unless a failover option, such as forwarding to a mobile, has been configured. Ask your provider about this before switching.
How do I know if my PBX supports SIP trunking?
Most PBX systems installed in the last decade support SIP, but it’s worth having a provider confirm compatibility with your specific model and version before signing up.
Keep what works, fix what doesn’t
SIP trunking gives Australian small businesses a practical way to move off the ageing copper network without replacing a phone system that’s otherwise doing its job. It’s often the lower-disruption, lower-cost option for businesses not ready for a full cloud PBX overhaul.
Nexgen has helped Australian businesses migrate their phone systems for 17 years, with local support and a managed porting process from start to finish. Get in touch with our team or call 1300 020 402 to find out whether SIP trunking is the right fit for your setup.
