SIP Trunking Sydney | Connect Your VoIP Phone System
A SIP trunk is the connection that links your VoIP phone system to the public phone network, letting it make and receive external calls and carry your business numbers. It is the modern replacement for the physical phone lines that used to run into an on-site PBX. 4iT provisions and manages SIP trunking for businesses across Greater Sydney, alongside the phone platform it connects to.
Sydney MSP
Greater Sydney, NSW
- Microsoft Partner
- Sophos Partner
- Ubiquiti Partner
ISDN out
legacy lines retiring
By channels
not by user count
Port across
keep your numbers
Key facts
- A SIP trunk carries calls between your phone system and the public phone network over the internet.
- It replaces the fixed phone lines (ISDN and analogue) that the legacy Australian phone network is retiring.
- SIP trunks are sized by the number of simultaneous calls (channels) you need, not by staff count.
- Your business numbers, including 1300 and 1800, live on the SIP trunk and remain portable.
- For an SMB, SIP trunking typically costs from around AU$15 to AU$60 per month plus call charges, depending on channels and numbers.
What is SIP trunking?
SIP trunking is the service that gives your phone system a way to reach the outside world. Your VoIP platform handles calls internally, but to call a customer or receive a call from one, it needs a connection to the public phone network. A SIP trunk is that connection, delivered over the internet rather than over physical lines, and it is what carries your phone numbers.
The name comes from the old world: a “trunk” was the bundle of phone lines running into a business. A SIP trunk does the same job using SIP, the protocol VoIP uses, so instead of physical lines you have a virtual connection sized to your needs. It is the piece that sits between your phone platform and your carrier, and it is easy to overlook until you realise no VoIP system can make an external call without one.
How many SIP channels does a business need?
The number of channels you need is the number of calls you expect to have happening at once, not the number of staff. A SIP channel carries one simultaneous call, so a business of 30 people that never has more than ten calls active at the same time needs around ten channels, with a little headroom. Sizing this correctly is where money is saved or wasted.
Over-provision and you pay for channels you never use; under-provision and calls get blocked at busy times. We size the trunk from your actual call patterns, looking at how many calls genuinely overlap during your busiest periods. This concurrency-based sizing mirrors how platforms like 3CX license, which makes the two fit together neatly.
Why move from phone lines to SIP trunks?
The first reason is that you may not have a choice for much longer. Australia’s legacy phone network, including ISDN, is being retired, so traditional business phone lines are disappearing. SIP trunking is the replacement, and moving on your own schedule is far better than being forced off at short notice.
The second reason is cost and flexibility. SIP trunks are cheaper than maintaining banks of physical lines, call rates are typically lower, and changing capacity is a configuration adjustment rather than a carrier installation. Combined with a hosted phone platform, SIP trunking removes the last piece of on-site phone hardware and turns your whole phone setup into a flexible, managed service.
Frequently Asked Questions
Yes. Your existing Australian numbers, including geographic, 1300, and 1800 numbers, port onto the SIP trunk and stay yours. Porting typically takes 10 to 20 business days through the losing carrier, and we manage the process so your numbers keep working right up to the cutover. You are not forced to change numbers when you move to SIP.
Yes, on a properly provisioned connection. SIP trunk reliability depends on your internet connection and network setup, which is why we configure quality-of-service and, for phone-critical businesses, recommend a backup connection. Calls can also fail over to mobiles or another route if the primary connection drops, so a single outage does not cut you off.
SIP trunking works with any modern VoIP phone system, including 3CX and other hosted platforms. It is a standards-based service, so it is not tied to one vendor. We make sure the trunk is configured correctly for your specific platform, since the details of registration and call routing vary between systems even though the underlying service is the same.
For an SMB, SIP trunking typically runs from around AU$15 to AU$60 per month depending on the number of channels and numbers, plus call charges that are usually lower than legacy line rates. The exact figure depends on your concurrency and call volume, so we size and quote it against your actual usage rather than a flat rate.
If you are still on ISDN or analogue phone lines, or setting up a new VoIP system and need it connected to the outside world, we are happy to size and provision the right SIP trunk and port your numbers across.
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