The short version
- A SIP trunk connects a phone system you control (on-premises, self-hosted or a Teams SBC) to the public phone network over the internet.
- You pay for channels (concurrent calls) and usage, and you can usually bring your existing numbers.
- It suits businesses that want to run their own PBX (3CX, FreePBX/Asterisk and similar) or use Teams Direct Routing.
- You take on more responsibility: security, firewall configuration, failover and emergency addresses.
How it works
SIP (Session Initiation Protocol) is the signalling language almost all VoIP uses to set up, change and end calls. A SIP trunk is an account with a provider that lets your phone system send and receive calls to the regular phone network using SIP. The audio itself travels separately as RTP packets.
Desk phones / softphones
│
Your PBX (on-site, VM or cloud server)
│ SIP + RTP over internet or private link
SIP trunk provider
│
Public phone network (landlines & mobiles)Compare that with a hosted cloud PBX, where the provider runs the PBX and you just plug in phones. With a SIP trunk, the PBX is yours.
When a SIP trunk makes sense
- You already own a capable IP PBX and want to replace ISDN or analogue lines feeding it.
- You want full control of the system: custom call flows, integrations, hosting location.
- You're connecting Microsoft Teams via Direct Routing.
- You have many users and a technical team, where per-channel pricing beats per-user hosted pricing.
- You need to connect specialised equipment like contact centre platforms, diallers or legacy systems.
What to compare between providers
| Factor | What to ask |
|---|---|
| Channels | How many concurrent calls are included, and how much is each extra? Can you burst above it? |
| Call rates | Local, national, mobile, 13/1300, international. Are calls per minute or per second? |
| Number porting | Can they port all your numbers, including 1300/1800 and number ranges (DID blocks)? |
| Codecs | G.711 A-law at minimum; is G.722 or Opus supported end to end? |
| Security | TLS and SRTP support? IP-based authentication? Spending caps and fraud alerts? |
| Redundancy | Multiple points of presence in Australia? Automatic failover to a mobile or another number if your site goes down? |
| Emergency calls | How do they handle 000 and the emergency address for each number? |
| Compatibility | Are they certified or documented for your PBX (3CX, FreePBX, Teams Direct Routing via your SBC)? |
Sizing: how many channels do you need?
A channel carries one call. A common starting point for an office is one channel for every three to four staff who use the phone regularly, more for reception-heavy or call centre teams. Look at peak concurrent calls in your current system's reports if you can. Each channel needs roughly 87 kbps each way on G.711, so 10 concurrent calls need around 0.9 Mbps of upload, before any other traffic. Our calculator works this out for you.
Resilience: plan for the internet going down
- Ask the provider to divert inbound calls automatically to mobiles or another site when your trunk is unreachable.
- Use a router with 4G/5G failover for the PBX's internet connection.
- Keep the PBX, router and NBN equipment on a UPS.
- Consider hosting the PBX in a data centre or cloud so a single office outage doesn't take everything down.
Security is on you
A self-managed PBX connected to a SIP trunk is exactly what toll fraudsters hunt for. Lock down SIP access to known addresses, set international call restrictions and spending caps, and keep the PBX patched. Work through our VoIP security checklist before you go live.