What is a SIP trunk? It’s the modern replacement for the bundle of phone lines that used to come into your building. Instead of physical copper or ISDN circuits, a SIP trunk delivers your phone numbers and call capacity over your internet connection, connecting your phone system to the public telephone network. Same calls, same numbers – just carried as data.
How does a SIP trunk work?
SIP (Session Initiation Protocol) is the standard that sets up and tears down voice calls over the internet. When someone dials your number, your provider routes the call over the internet to your phone system – a PBX like 3CX or Yeastar – which rings the right desk phone, app or queue. Outbound calls travel the reverse path. Your “trunk” is simply the authenticated connection between your PBX and your provider’s network.
Channels vs phone numbers – the distinction that matters
People mix these up constantly:
- Channels (also called lines or simultaneous calls) = how many conversations can happen at once. A 10-channel trunk supports 10 concurrent calls.
- Numbers (DIDs) = the phone numbers people dial. You can have 100 numbers on a 10-channel trunk.
You pay mostly for channels, so sizing them right matters: a 20-person office rarely needs more than 6-10 channels, because not everyone is on the phone at once.
What happened to ISDN?
Australia’s ISDN network – the old standard for business phone lines – has been switched off as part of the NBN transition (we covered the shutdown in our post on the end of ISDN). SIP trunking is what replaced it: cheaper per channel, faster to scale (add channels in software, not site visits), and delivered over any decent internet service.
What do you need to run SIP trunks?
- A phone system that speaks SIP – any modern PBX, hosted or on-site.
- A solid internet connection – each call uses roughly 100 kbps; even a modest NBN plan carries a small office’s calls, but quality-of-service settings matter. Our SIP and firewall requirements guide covers the technical checklist.
- Your numbers – new ones, or your existing numbers ported across. How porting works is covered in depth in our guide to number portability in Australia, and the practical steps are in porting to Tronic.
Why businesses switch
- Cost: SIP channels cost a fraction of what ISDN did, and calls between your sites are free.
- Flexibility: scale channels up for a campaign, down after; numbers from any Australian region without an office there.
- Continuity: if your office loses power or internet, calls redirect to mobiles or another site in seconds.
Tronic SIP trunks carry business calls across Australia on local infrastructure, with support included – and they pair with any SIP-capable phone system, not just ours.
Frequently asked questions
Is a SIP trunk the same as VoIP?
VoIP is the broad technology of carrying voice over the internet. A SIP trunk is the specific service that connects your phone system to the public phone network using VoIP. If VoIP is driving, SIP trunking is the highway on-ramp.
How many SIP channels do I need?
Count your realistic peak simultaneous calls, not your staff. Most offices run happily on one channel per two to three staff. A call centre needs closer to one per agent plus queue headroom.
Can I keep my existing phone numbers?
Yes – Australian numbers, including 1300 and 1800 numbers, can be ported to a SIP provider. Never cancel the old service before the port completes.
What internet speed do I need for SIP trunks?
Each call uses about 0.1 Mbps, so bandwidth is rarely the problem – consistency is. A stable connection with QoS prioritising voice beats a fast but jittery one.
Sizing up a switch?
Read how Tronic SIP trunks work, check the porting process for your numbers, or send us your current phone bill and we will map out channels, numbers and costs for you – no obligation. Sales: 1300 788 718.
