What Is SIP Trunking?
A SIP trunk is a virtual phone line that connects your phone system to the public telephone network (PSTN) via the internet, using the Session Initiation Protocol (SIP). It replaces the physical copper lines — ISDN BRI, ISDN PRI, or analogue PSTN lines — that traditional phone systems use to make and receive calls.
Think of it this way: where a traditional phone line is a physical cable running from your office to the telephone exchange, a SIP trunk is a logical connection that travels over your broadband internet connection.
Why Is Australia Moving to SIP Trunking?
The migration to SIP trunking in Australia is being driven by two converging forces:
The NBN rollout has replaced the copper network that ISDN and PSTN services depend on. As the NBN reaches each area, Telstra and other carriers begin the process of migrating customers off legacy services. In many areas, ISDN services are no longer available for new connections.
The PSTN switch-off — Telstra has announced the progressive decommissioning of the PSTN, with a target of completing the transition by 2025. Businesses that haven't migrated will eventually have no choice.
How SIP Trunking Works
A SIP trunk connects your existing phone system (whether a legacy PABX, a 3CX system, or another IP PBX) to a SIP trunk provider. The provider routes your calls to and from the PSTN.
The key components are:
- 1Your phone system — This can be an IP PBX like 3CX, or a legacy PABX with a SIP gateway
- 2Your internet connection — SIP trunks travel over your broadband connection
- 3The SIP trunk provider — A carrier that provides the connection to the PSTN
- 4Your phone numbers — Your existing numbers are ported to the SIP trunk provider
The Cost Comparison
Here is a typical cost comparison for a 10-person business with 4 concurrent calls:
| Service | Traditional ISDN | SIP Trunking |
|---|---|---|
| Line rental (4 channels) | $240/month | $60/month |
| Local calls | $0.22/call | Included |
| National calls | $0.30/min | $0.03/min |
| Mobile calls | $0.45/min | $0.09/min |
| International calls | $1.20/min | $0.08/min |
| **Estimated monthly total** | **$380–$520** | **$80–$120** |
The savings are substantial — typically 60–75% on line and call costs.
Key Features of Modern SIP Trunking
Unlimited concurrent calls — Unlike ISDN, which has a fixed number of channels, SIP trunks can scale dynamically. You only pay for the channels you need.
Geographic flexibility — You can have a Sydney 02 number, a Melbourne 03 number, and a Brisbane 07 number — all routing to your Toowoomba office. This is invaluable for businesses that want a local presence in multiple cities.
Number portability — Your existing phone numbers port to the new SIP trunk. You keep the numbers your customers already know.
Redundancy — SIP trunks can be configured with failover to a secondary provider or to mobile numbers, ensuring business continuity if your primary internet connection fails.
Call recording — Most SIP trunk providers offer call recording as an optional add-on, which is useful for compliance and quality assurance.
Choosing a SIP Trunk Provider in Australia
Key factors to consider when selecting an Australian SIP trunk provider:
- Local number portability — Can they port your existing numbers?
- Call quality — Do they operate their own network or resell another carrier's service?
- Pricing model — Per-channel rental plus per-minute calls, or unlimited calls?
- Redundancy — Do they have multiple points of presence and automatic failover?
- Support — Is support available during business hours? After hours?
- Contract terms — Month-to-month or locked-in contracts?
ClearCall partners with leading Australian SIP trunk providers to offer our clients competitive pricing and reliable service. We manage the entire process — from provider selection and number porting to configuration and ongoing support.
Is SIP Trunking Right for Your Business?
SIP trunking is suitable for any business with a reliable internet connection and an existing phone system. If you are currently on ISDN or PSTN lines, the switch to SIP trunking will almost certainly save you money and future-proof your communications infrastructure.
Contact ClearCall for a free SIP trunking assessment and cost comparison.