What a SIP trunk actually is
Strip away the acronyms and a SIP trunk is a simple idea: it is a set of phone lines that arrives over your internet connection instead of over copper wire or an ISDN circuit. "SIP" stands for Session Initiation Protocol - the standard language that phone systems use to set up, ring and tear down calls across the internet. The "trunk" is the old telephony word for a bundle of lines running into your business.
So a SIP trunk gives you two things at once: your business phone numbers (including numbers you port across), and a pool of concurrent call paths called channels. It plugs into a phone system you already run - your PBX - and from the outside nothing changes. Customers dial the same numbers, your team answers on the same handsets or apps, and calls sound the same. What has changed is what is underneath: software and data rather than dedicated voice wiring you pay for whether you use it or not.
This is where people confuse SIP trunking with "VoIP" in general. VoIP simply means voice carried over the internet - it is the broad category. A SIP trunk is one specific, business-grade way of doing VoIP: carrier-delivered lines and numbers that feed a PBX. If you want the wider picture of how internet voice works, our business VoIP guide for Australia covers the fundamentals.
The one-sentence version
A SIP trunk replaces your physical phone lines with virtual ones over the internet, delivering your numbers and concurrent-call capacity into a phone system you keep running yourself.
Why SIP exists: the ISDN shutdown
SIP trunking is not a shiny new toy - for a lot of Australian businesses it is now simply the only option. For decades the workhorse for multi-line business phone service was ISDN (and its bigger sibling, ISDN PRI). Then Telstra retired the technology: ISDN was fully decommissioned by mid-2022. Services stopped being sold, then stopped being supported, and the copper-era voice network was switched off.
If your business is still limping along on ISDN, PRI, or a rack of legacy line cards, migration is not optional - it is overdue. SIP trunking is the mainstream replacement precisely because it maps onto ISDN so cleanly: where you once had, say, 10 ISDN channels, you now have 10 SIP channels doing the same job over your internet link. Your numbers come with you.
The upside is that SIP fixes ISDN's most annoying limits. Adding or removing capacity used to mean ordering physical lines and waiting weeks; with SIP a channel is a software setting your provider can change quickly. And because you are no longer paying rental on fixed copper channels, the economics usually improve sharply.
2022
Year Telstra's ISDN was fully decommissioned
30-60%
Typical monthly voice-spend cut moving off ISDN (illustrative)
~100 Kbps
Bandwidth per concurrent SIP call
SIP trunk vs cloud PBX: how to choose
This is the decision that trips up most buyers, so it is worth being precise. A SIP trunk and a cloud (hosted) PBX are not competitors doing the same job at different prices - they solve different halves of the problem.
A SIP trunk feeds a PBX you already own and operate. The brains of your phone system - the call routing, extensions, voicemail, IVR - still live in your equipment, whether that is an on-premise box in a comms cupboard or a virtual PBX such as 3CX running on your own server or cloud instance. The SIP trunk just supplies the outside lines and numbers.
A cloud PBX replaces that phone system entirely. The routing, features and numbers all live in the provider's network, and your team simply uses apps, desk phones and a web console. There is no PBX for you to patch, license or babysit.
✓ Choose a SIP trunk if you have a capable, recent PBX (or a 3CX deployment) that does what you need and you mainly want to modernise the lines behind it.
✓ Choose a cloud PBX if your hardware is ageing, you are tired of managing servers and licences, or you want built-in AI, mobile apps, SMS, video and CRM integrations without bolting them on.
◐ Consider both in a transition: run a SIP trunk into your existing PBX now, then move fully to cloud when the hardware retires.
| Question | SIP Trunk | Cloud PBX |
|---|---|---|
| Who runs the phone system? | You do (on-prem or 3CX) | The provider does |
| On-site hardware to maintain? | Usually yes | No |
| Features (AI, apps, SMS, video) | Whatever your PBX supports | Included in the platform |
| Keep your numbers? | Yes, via porting | Yes, via porting |
| Best fit | Capable PBX worth keeping | Retiring hardware / want all-in-one |
| Upgrades & patching | Your responsibility | Handled for you |
If you think you are leaning toward retiring the box, weigh it up properly first - our guide to hosted PBX vs on-premise PBX in Australia walks through the total-cost and control trade-offs in detail. And if you are running 3CX today, the practical steps are covered in our 3CX to VOCPhone migration guide.
Sizing channels, bandwidth and concurrent calls
The single biggest mistake in a SIP migration is buying the wrong number of channels - usually because someone sized it against staff headcount. A channel is one simultaneous call, not one person. In a 40-person office it is rare for more than a handful of people to be on the phone at the exact same moment, so that business might only ever need 10-12 channels.
The reliable way to size a trunk is to look at your own call data. Pull a report of your busiest hour over a normal month and find the peak number of concurrent calls, then add a little headroom for growth and unusual spikes. Provisioning is done in software, so if you consistently hit the ceiling you can add channels quickly - you are not waiting on a physical line install.
- Find your peak. Check historical call reports for the highest number of simultaneous calls during your busiest hour.
- Add headroom. Give yourself a small buffer above that peak for growth, seasonal spikes and campaigns.
- Check your internet. Budget around 100 Kbps up and down per concurrent call, and make sure your connection has that capacity with room to spare.
- Prioritise voice. Use quality-of-service (QoS) settings so calls take priority over downloads and backups on the same link.
Bandwidth rule of thumb
Each concurrent call needs roughly 100 Kbps in each direction. Twelve channels is therefore only about 1.2 Mbps of voice traffic - trivial for most NBN and business fibre connections, but it must be reliable, low-latency bandwidth, not just a big headline speed. Jitter and packet loss matter far more to call quality than raw megabits.
If you do hit call-quality gremlins after a move - one-way audio, dropouts, registration failures - most have well-known causes. Our guide to troubleshooting VoIP and SIP problems runs through the usual suspects and fixes.
Compliance: Triple Zero, numbering and porting
Voice is a regulated service in Australia, and moving to SIP does not exempt you from the rules - it just changes who is responsible for meeting them. A compliant provider must satisfy the Telecommunications Numbering Plan and the Triple Zero (000) and IPND obligations. In plain terms, that means your numbers are legitimate and portable, and that emergency calls are handled correctly.
Triple Zero deserves special attention. On old copper, your location was baked into the physical line. SIP is not location-bound - the same trunk can theoretically be used anywhere with internet - so your registered emergency service address has to be kept accurate in the provider's records and the Integrated Public Number Database (IPND). After any migration or office move, confirm that address is current so that if someone dials 000, help is sent to the right place.
Don't assume 000 "just works"
Triple Zero must keep working through and after your migration, and the registered address behind each number must be correct. Make this an explicit checklist item with your provider - never a background assumption. This is one of the clearest reasons to use a compliant Australian carrier rather than an offshore or grey-market trunk.
Porting your existing numbers is the other piece. A reputable provider will manage the port for you, coordinate the cutover to minimise downtime, and keep your published numbers intact so customers never notice. Ask upfront how porting timeframes and any interim call-forwarding will be handled.
What SIP trunking costs in Australia
SIP trunking is popular for a very practical reason: it is usually a lot cheaper than what it replaces. Legacy ISDN charged rental on every channel - illustratively around $30-60 per channel per month - whether or not you used it, plus call rates on top. Drop that fixed rental and move to bundled or per-second call pricing and most businesses see monthly voice spend fall by roughly 30-60%.
Pricing for SIP trunks generally comes in three parts, and it pays to understand each before you compare quotes:
Channels
A monthly charge per concurrent call path. This is where right-sizing matters - pay for peak concurrency, not headcount.
Numbers
A small monthly fee per phone number or number range you hold, including ported numbers.
Call usage
Either bundled/unlimited plans or per-second rates for local, national, mobile and international calls.
Watch for the traps that erode the saving: setup or porting fees, minimum contract terms, USD-priced plans exposed to exchange-rate swings, and charges for "extras" that should be standard. A transparent Australian provider quotes in Australian dollars and spells out what is included. For a broader view of what a phone system really costs once everything is added up, see our breakdown of business phone system costs in Australia.
Choosing a SIP trunk provider
Once you have decided a SIP trunk (rather than a full cloud PBX) is the right shape, the provider you pick matters more than the headline per-channel price. Voice is only as good as the network it runs on, and support is only useful if you can actually reach it. A few criteria separate a serious carrier from a middleman:
Owns its network
Many "providers" simply resell someone else's trunks, adding a layer between you and any problem. VOCPhone owns and operates its own network, so call quality and fixes are in our hands - not a wholesaler's queue.
Australian support
When a trunk misbehaves you want a local human, not an offshore script. Look for genuine Australian 24/7 support and Australian data hosting for data-sovereignty peace of mind.
Compliance built in
Confirm numbering-plan compliance, correct Triple Zero handling, IPND registration and managed number porting - non-negotiables for any legitimate carrier.
Room to grow
Channels you can add in software, transparent AUD pricing, and a clear path to cloud PBX features later if you decide to retire the hardware.
The right question is not just "how cheap is a channel?" - it is "who actually answers when a call drops at 4pm on a Friday?" With a network we own and an Australian team, that answer is us.
- The VOCPhone team
Whether you keep your PBX and run a SIP trunk into it, or eventually move the whole system into the cloud, the goal is the same: reliable calls, sensible pricing and a provider who is accountable end to end. If you are weighing up going fully cloud, our explainer on what a cloud phone system is is a good next stop, and if Microsoft Teams is part of your world, see how Teams Phone and Direct Routing work in Australia.
Frequently Asked Questions
What is a SIP trunk in simple terms?
A SIP trunk is a virtual set of phone lines delivered over your internet connection instead of copper or ISDN. It carries your business phone numbers and concurrent calls to and from your existing phone system (PBX), so you can make and receive calls exactly as before - just over data rather than dedicated voice wiring.
What is the difference between a SIP trunk and a cloud PBX?
A SIP trunk simply feeds calls into a PBX you already own and run - on-premise or virtual, such as 3CX. A cloud (hosted) PBX replaces that PBX entirely: the phone system itself runs in the provider's network and you just use apps, handsets and a web console. Choose a SIP trunk if you have a capable PBX you want to keep; choose a cloud PBX if you want to retire on-site hardware and get features like AI, apps, SMS and video without managing servers.
Is ISDN still available in Australia in 2026?
No. Telstra fully decommissioned its ISDN network by mid-2022, so ISDN and PRI services are no longer sold or supported. Any business still running on ISDN must migrate, and SIP trunking is the standard replacement - it delivers the same channels and numbers over an internet connection.
How many SIP channels do I need?
Size channels to your peak concurrent calls, not your headcount. A channel is one simultaneous call, so a 40-person office where 10 people are ever on the phone at once typically needs around 10-12 channels. Check historical call reports for your busiest hour and add a little headroom. As a rough guide each concurrent call uses about 100 Kbps of upload and download.
Will Triple Zero (000) still work after I move to SIP?
Yes, provided you use a compliant Australian provider. Carriers must meet the Telecommunications Numbering Plan and Triple Zero and IPND obligations, which means your 000 calls are routed correctly and your service address is registered. Always confirm your registered emergency address is current after any move, because SIP is not location-bound the way copper was.
How much can SIP trunking save compared with ISDN?
Most businesses cut monthly voice spend by roughly 30-60% moving off ISDN or PRI, because they drop per-channel line rental (illustratively around $30-60 per channel per month on legacy ISDN) and often move to bundled or per-second call rates. You also avoid the cost of physically re-wiring when you add or remove lines, since channels are provisioned in software.
