Most conversations about contact centres focus on the software: the queues, dashboards, CRM integration, and, lately, AI. But every one of those calls still has to travel in and out of the business, and that is the job of the SIP trunk. When the trunk is sized and set up well, nobody notices it. When it isn’t, customers hear busy tones, outbound campaigns stall, and agents sit idle with a full queue they cannot reach.
Here is how SIP trunks fit into a modern contact centre, and what to check so yours keeps up.
What does SIP trunking do in a contact centre?
SIP trunking connects a contact centre’s phone platform to the public phone network over a data connection, carrying every inbound and outbound call. It provides the call capacity, phone numbers and caller ID that the queues, IVR, dialler and recording tools depend on to work.
A simple way to think about it: the contact centre platform is the brain, and the trunk is the set of lines. The platform decides which agent gets the call, plays the hold music and records the conversation. The trunk decides how many calls can happen at once and which numbers customers see and dial.
That split matters because the two are often bought separately. Some cloud phone systems include calling as part of the package. Others, along with most on-premises platforms, need you to bring your own carrier. Either way, the SIP trunking underneath shapes how much the platform can actually handle.
Five ways SIP trunks support contact centre work
Scaling for peaks
Contact centres rarely run at a steady pace. Energy retailers get flooded during outages, insurers during storm season, and retailers before Christmas. With the old ISDN services, capacity came in fixed blocks that took weeks to add.
SIP channels are added in software, often within a day. Some providers also offer burst capacity, where you can go above your usual channel count for short spikes. That means you can plan for your busy season instead of paying for it all year.
Inbound numbers and routing
Your 1300, 1800 and 13 numbers point at the trunk, along with any local numbers. Many contact centres use a different number for each campaign, product line or state, then use the number the customer dialled to send the call to the right queue. A 1300 number for sales and a separate one for support lets you report on each without asking callers to press anything.
Outbound calling and caller ID
For outbound teams, the trunk controls the number customers see when you call. That number is your CLI (Calling Line Identification), and in Australia, it matters more than it used to. Under the ACMA-registered industry code C661:2022 Reducing Scam Calls and Scam SMs, telcos identify, trace and block scam calls, and calls presenting invalid or spoofed numbers can be blocked before they reach anyone.
In practice, that means presenting a number your business actually holds and is authorised to use, and checking with your provider before you present numbers held with another carrier. Outbound teams also need to work within the Do Not Call Register rules.
Remote and hybrid agents
With a cloud contact centre, the trunk connects at the platform, not at each agent’s desk. Agents can log in from the office, home or a second site, and every call still goes out on the same numbers and shows up in the same reports.
Call recording and AI tools
Recording, transcription and speech analytics all work from the audio the trunk delivers. Many trunks support more than one codec, the format used to compress voice. G.711 carries fuller audio and uses about 100 kbps per call, while G.729 uses less bandwidth but compresses the voice more heavily. If you rely on call recording for quality checks or AI transcription, it is worth asking which codec your calls use, as heavier compression can make recordings harder to understand and transcripts less accurate.
How many SIP channels does a contact centre need?
A contact centre needs enough SIP channels to cover its busiest moment, which is the number of agents on calls plus every caller waiting in a queue or using the IVR at that moment. Callers on hold still use a channel. Outbound diallers and transfers to outside numbers can also push channel use well beyond the number of agents.
This is where many contact centres get caught out. The table below shows where channels go.
| What’s happening | Channels it uses | Sizing tip |
| Agent talking to a customer | 1 per call | Start with your peak agent headcount. |
| Caller waiting in a queue | 1 per caller | Check your longest queue at the busiest hour. |
| Caller in the IVR menu or self-service | 1 per caller | Add your typical IVR load at peak. |
| Predictive dialler making outbound calls | Often more than 1 per agent | Ask your dialler vendor for its dial ratio. |
| Call transferred to an outside number | Often 2 (one in, one out) | Count transfers to mobiles, other offices or partners. |
Here is a simple example. A team of 20 inbound agents is fully busy at 10 am on a Monday, with 10 callers in the queue and 5 more working through the IVR. That moment needs about 35 channels, and a buffer of a few more makes sense. If that team only had 20 channels, callers 21 to 35 would get a busy tone, even though the queue was set up to hold them.
Outbound is where the gap tends to be widest. A predictive dialler set to dial three numbers for every free agent could need around 30 channels for a team of 10, before any transfers are counted.
Choosing how the trunk and platform fit together
| Setup | Where the trunk connects | Tends to suit | Things to watch |
| On-premises phone system with SIP trunks. | Your office, over your internet link. | Larger single-site teams with in-house IT. | Office internet becomes a single point of failure without a backup link. |
| Cloud contact centre with your own SIP trunk. | The platform’s data centre. | Teams that want to choose their carrier and keep their numbers. | Two suppliers to coordinate when there is a fault. |
| Cloud platform with calling included. | The platform’s data centre. | Smaller or fast-growing teams that want one bill. | Less control over rates, CLI options and carrier choice. |
The choice usually comes down to how much control you want and who will support it. Many Australian businesses moving off older phone systems start by comparing business VoIP solutions, then decide whether to keep the trunk separate or bundled. If you also run other IP telephony services, such as Microsoft Teams calling, map out the whole setup before you choose.
SIP trunking in Australia: What contact centres should check
Redundancy and failover
Ask what happens if your internet link or the provider’s network goes down. Good setups route calls to a backup site, mobile numbers or a recorded message rather than simply failing. For on-premises systems, a second internet connection (such as a 4G or 5G backup, or fibre from a different provider) is worth considering. It is also worth testing failover before your busy season rather than finding out during it.
Where calls and recordings are handled
For finance, health and government work in particular, it matters where calls are processed, and recordings are stored. Ask whether the trunk and platform use Australian points of presence and onshore storage. This helps call quality, too, as calls routed offshore and back can pick up delay.
Internet bandwidth for on-premises setups
If the trunk connects at your office, plan for around 100 kbps in each direction per call on G.711. The 35-channel example above would need about 3.5 Mbps of upload and download set aside for voice at peak. Quality of Service (QoS) on your router keeps that voice traffic ahead of everything else.
Emergency calls for remote agents
Agents working from home can dial 000 from their work phone, so make sure each number has the right registered address. Staff working away from that address should use a mobile in an emergency.
Getting the foundation right
Before your next busy period, pull the call data from your busiest hour of the past few months. Note the highest number of calls happening at once (including callers in queues and the IVR), your outbound dial ratio, and the transfers that go to outside numbers. Compare that with your current channel count and test your failover.
If the numbers are close, a small increase in channels or a burst option may be all you need. If they are not, our team at Com2 can review your setup and suggest a trunk and platform mix that suits your contact centre.