Glossary
SIP trunking
Virtual phone lines connecting a business's phone system to the public telephone network over the internet, using the SIP protocol.
also known as: SIP trunk, SIP line
A SIP trunk is a virtual phone line that connects your phone system to the public telephone network using the [[sip]] protocol over the internet - replacing traditional [[hosted-pbx|ISDN/PSTN copper]] lines.
How it works
Your phone system speaks SIP to a SIP trunk provider’s servers. The provider’s servers connect to the public phone network and route calls. From the outside-world caller’s perspective, your business has a phone number. Behind the scenes, the call flows over the internet from your office to the provider, then onto the public network.
Channels - sizing your trunk
SIP trunks are measured in channels (concurrent calls). A 10-channel trunk handles 10 calls at once. Sizing for Australian business:
- Small business (5–20 users) - 4–8 channels typically.
- Medium business (20–80 users) - 8–16 channels.
- Contact centre - usually 1 channel per agent.
Adding more channels later is a config change - no truck roll, no installer visit.
Costs
Two components:
- Per-channel monthly fee - typically AUD $5–15 per channel per month.
- Per-minute call charges - local Australian landline $0.02–0.10/min; mobile $0.10–0.25/min; international varies.
Inbound on standard numbers is usually free. 1300/1800 has receive charges.
Cloud Phone System Australia bundles Australian SIP trunking into the 3CX managed-service deployment - you don’t manage a separate trunk invoice.
Why SIP trunking replaced ISDN
- Cheaper - wholesale-rate calls without telco infrastructure tax.
- Scalable - add channels in software.
- Flexible - channels can be reallocated between sites or roles.
- Resilient - failover trunks possible; multi-path internet for redundancy.
- Modern - works with cloud PBXs, mobile apps, video conferencing.
- Australian policy - ISDN is being decommissioned by 2030.
See also
- [[sip]] - the underlying protocol
- [[hosted-pbx]] - phone systems that use SIP trunks
- [[did-number]] - direct inward dialling numbers via SIP
- [[sbc]] - Session Border Controller for SIP security
