Connecting a SIP trunk to 3CX
How to add and configure a SIP trunk in 3CX so your business can make and receive external calls.
A SIP trunk is what carries your calls between 3CX and the outside world. Get it connected and configured correctly and your team can dial out, take calls on your business numbers, and route everything through 3CX the way you want. Get it wrong and you'll see one-way audio, failed registrations or calls that simply won't connect.
This guide walks through what a SIP trunk does in 3CX, the information you need from your provider, and how to add and test the trunk so calls flow both ways. We keep it conceptual on purpose, because exact menus shift between 3CX versions, but the right terms and the right order never change.
What a SIP trunk actually does
Your old phone line plugged into a wall. A SIP trunk does the same job over the internet: it's the connection between your 3CX phone system and a voice carrier, and it's what lets you make and receive calls on the public phone network.
One trunk can carry many simultaneous calls. The number of concurrent calls (often called channels) is what you're really buying, not a count of phone numbers. Your numbers, including any DIDs and 1300 or 1800 numbers, are delivered over that same trunk and then routed inside 3CX. If you're still fuzzy on the underlying technology, our plain-English explainer on SIP trunks explained is a good ten-minute read.
What you'll need before you start
Half the pain of a SIP trunk setup comes from missing details. Gather these from your carrier first:
Trunk type
most Australian trunks are registration-based (3CX logs in with a username and password) or IP-based (the carrier authorises your fixed public IP address). You need to know which.
SIP credentials
the SIP server or proxy address, port, username/authentication ID and password.
Your numbers
the main number plus any DID range, in the exact format the carrier sends them (for example +61 vs 0 prefix).
Concurrent call limit
how many simultaneous calls the trunk allows, so you don't oversell your own lines.
The caller ID format
the carrier expects on outbound calls.
A solid internet connection matters as much as the credentials. Voice needs low latency and steady upload bandwidth, so a business-grade NBN or fibre service is worth having sorted before go-live.
Adding the trunk in 3CX
In the 3CX Admin Console you'll add the trunk under SIP Trunks. The cleanest path is to choose your carrier from the provider list if it's there, because 3CX pre-fills the technical settings it knows about. If your carrier isn't listed, use the generic SIP trunk template and enter the details by hand.
Create a new SIP trunk and select your provider, or the generic template, then enter the SIP server address.
Enter the main trunk number and your authentication details (username and password for a registration trunk, or confirm your public IP for an IP-based trunk).
Set the number of simultaneous calls to match what the carrier sold you.
Confirm the trunk registers — 3CX will show the trunk status as registered or online when the credentials and network path are correct.
If the trunk won't register, the usual culprits are a wrong password, a blocked SIP port on the firewall, or the carrier not yet provisioning your account. Check status before moving on; routing rules can't work over a trunk that isn't up.
Routing calls in and out
A registered trunk is only half the job. 3CX needs to know what to do with calls.
Inbound: create Inbound Rules so each DID lands where it should — a specific extension, a ring group, a call queue, or a digital receptionist. You can route differently inside and outside office hours, so after-hours calls go to voicemail or an answering message instead of ringing an empty office.
Outbound: create Outbound Rules that decide which trunk a call uses based on what's dialled, and set the outbound caller ID so the right number shows on the recipient's phone. Get the caller ID right or your customers see a number they don't recognise.
Test, then secure it
Before you call it done, test both directions properly:
- Dial out to a mobile and confirm two-way audio and the correct caller ID.
- Call each published number from outside and confirm it lands where the inbound rule says.
- Test a few calls at once to confirm the channels behave as expected.
If you hear audio in only one direction, that's almost always NAT or firewall related rather than a trunk fault — our guide on one-way audio and NAT issues covers the fixes. Once calls work, lock the system down: an open SIP trunk is a target for toll fraud. Use 3CX's anti-fraud and security limits, restrict outbound destinations you don't call, and read securing 3CX against toll fraud. If uptime is critical, plan for internet failover so a dropped connection doesn't take your phones with it.
SIP trunks reward careful setup and punish guesswork. If you'd rather have it done right the first time, this is exactly the work we do for Melbourne businesses every week — call our team on 1300 747 266 or grab a free bill review and we'll sort the trunk, the numbers and the routing for you.
What we see installing this in Melbourne
On Melbourne SME sites the trunk itself is rarely the problem — the connection underneath it is. Plenty of small offices are still on a cheap NBN plan with 20 Mbps upload, and once five or six concurrent calls hit during a busy morning, audio gets choppy long before the channels run out. We test on the actual office line before cutover, because the result is often very different from a lab test.
The other recurring trap is a consumer-grade router with SIP ALG quietly enabled. It mangles SIP packets and you get registrations that drop or one-way audio on external calls. We turn it off and set proper QoS.
With PSTN switch-off pressure on, owners often want it done yesterday. We hold the porting window open, keep the old service live until the new trunk is proven, and stay onsite the morning after cutover.
Frequently asked questions
How many SIP channels does my business need?
Count the maximum number of calls you'd realistically have running at the same time, then add a little headroom. A small office where three or four people are on the phone at once rarely needs more than five or six channels. It's the concurrent calls that matter, not how many staff or numbers you have.
Can I keep my existing phone numbers when I move to a SIP trunk?
Yes. Your numbers are ported to the new carrier and then delivered over the SIP trunk and routed in 3CX. Done properly, porting happens without downtime — see our guide on porting numbers to 3CX and our number porting service.
Do I need a SIP trunk if I'm on hosted or cloud 3CX?
Yes. Whether 3CX is hosted in the cloud or on your own server, it still needs a SIP trunk to reach the public phone network for external calls. The trunk is what connects 3CX to a carrier either way.
Why does the trunk register but calls still fail?
Registration only proves the login works. Failed calls usually trace back to outbound rules sending calls down the wrong path, a caller ID the carrier rejects, or a firewall blocking the media (audio) ports while letting signalling through. Test outbound and inbound separately to narrow down which side is the problem.
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