SIP Trunking
SIP trunking is a way of connecting a phone system to the public telephone network over the internet, using the Session Initiation Protocol (SIP) to carry calls — replacing the physical ISDN lines businesses used for decades.
In plain terms, a SIP trunk is the modern, internet-based version of a phone "line" or "trunk". Instead of running copper or ISDN circuits into your building, you connect your phone system to a SIP trunking provider over your data connection.
It's closely tied to two older terms you may be searching for — SIP (the protocol itself) and trunks (the traditional bundles of phone lines) — so this guide covers all three.
This guide explains what SIP trunking is, what SIP and trunks mean, how it works, how it differs from ISDN and cloud phone systems, and what it means for Australian contact centres.
What it is
A virtual, internet-based phone "trunk" that connects your phone system to the telephone network using SIP, in place of physical ISDN lines.
Why it matters
It's the standard way businesses keep an existing phone system connected as ISDN is switched off — usually cheaper and far more flexible than the lines it replaces.
What this guide covers
What SIP and trunks are, how SIP trunking works, SIP trunking vs ISDN and cloud systems, the Australian switch-off, and choosing a provider.
What is SIP Trunking?
In plain English
SIP trunking replaces the physical phone lines that used to connect your phone system to the outside world with a virtual connection over the internet. Your existing phone system keeps working, but instead of ISDN circuits, calls travel as data using VoIP and the SIP protocol.
Each SIP trunk carries a number of simultaneous calls (called channels or sessions), and you buy as many as you need for your call volume — scaling up or down without new cabling.
SIP trunking is often the natural first step in modernising business voice: it lets you keep an on-site phone system (a PBX) you've already invested in, while moving the underlying connection to the internet.
What it is
A virtual, internet-based replacement for physical phone trunks, connecting your phone system to the network via SIP and VoIP.
What it isn't
It's not a full phone system or a contact centre platform. SIP trunking is only the connection; your phone system or cloud platform still handles the calls.
SIP and Trunks Explained
Two older terms sit behind "SIP trunking", and it helps to understand each on its own.
SIP (Session Initiation Protocol)
SIP is the signalling protocol that sets up, manages and ends calls over the internet. It's the "handshake" that connects two parties so voice (carried by VoIP) can flow between them. SIP is used for voice, video and messaging.
Trunks
A "trunk" is telephony's word for the connection that carries calls between your phone system and the wider network. Traditionally trunks were physical bundles of lines (such as ISDN); a SIP trunk is the virtual, internet-based equivalent.
Putting it together
SIP trunking simply means using SIP to deliver trunks over the internet. Where a business once ordered ISDN trunks from a telco, it now orders SIP trunks from a provider — same purpose, delivered over your data connection instead of dedicated copper.
How SIP Trunking Works
The flow is straightforward: your phone system connects to a provider's network over the internet, and SIP trunks carry your calls.
Your phone system
You keep your existing phone system (an IP-PBX, or a traditional PABX with a gateway) on site or in a data centre.
Connect to a provider
Instead of ISDN lines, the system connects to a SIP trunking provider over your internet or a dedicated data link.
Buy channels
You purchase the number of concurrent call channels you need — each channel is one simultaneous call in or out.
Make and take calls
Calls are set up by SIP and carried as VoIP, keeping your existing numbers and dial plan.
💡 Channels, not lines
With ISDN you bought fixed lines in blocks. With SIP trunking you buy channels (concurrent calls) and can add or remove them quickly, which makes it far easier to match capacity to demand — useful for contact centres with seasonal peaks.
SIP Trunking vs ISDN and Cloud Systems
SIP trunking sits between the old ISDN world and fully cloud-based phone systems. Knowing the difference helps you choose the right path.
SIP trunking vs ISDN
ISDN delivered trunks over dedicated physical lines; SIP trunking delivers them over the internet. SIP is typically cheaper, quicker to provision, and scales in single channels rather than fixed blocks — which is why it's the standard replacement as ISDN is retired.
SIP trunking vs cloud phone systems
SIP trunking keeps your own phone system and just replaces the connection. A hosted PBX or cloud contact centre replaces the phone system itself with a vendor-hosted service. SIP suits businesses keeping existing kit; cloud suits those modernising the whole platform.
The Australian Switch-Off
SIP trunking exists in Australia largely because of what's happening to the old networks.
ISDN is being retired
Telstra stopped selling new ISDN services in 2018 and has been disconnecting existing ISDN lines in the years since, as part of the broader NBN migration that is switching off the traditional copper network. For businesses that want to keep an existing phone system rather than replace it outright, SIP trunking is the standard way to stay connected once ISDN is gone.
The alternative is to move to a fully hosted or cloud phone system. Either way, the destination is VoIP over the NBN. This is general information, not telecommunications advice — confirm timelines for your services with your provider.
Choosing a SIP Trunking Provider
Providers vary in price, call quality, resilience and support, so it pays to compare against your requirements.
💡 What to look for
Key considerations include voice quality and network reliability, how channels are priced, number porting, failover and disaster-recovery options if your internet drops, fraud protection, and whether the provider supports your particular phone system. For a contact centre, capacity to burst channels during peaks is especially important.
💡 Compare providers
The Business Phone Systems section of the ACXPA Supplier Directory lets you compare SIP trunking and phone system suppliers, and the Contact Centre Technology category covers full platforms if you're modernising the whole operation.
Frequently Asked Questions About SIP Trunking
What is SIP trunking in simple terms?
SIP trunking is a way of connecting your phone system to the telephone network over the internet instead of using physical phone lines. It uses the Session Initiation Protocol (SIP) to set up calls, which are then carried as VoIP. In effect, a SIP trunk is the modern, internet-based replacement for the traditional ISDN lines a business used to order from a telco.
What is the difference between SIP and SIP trunking?
SIP (Session Initiation Protocol) is the signalling protocol that sets up, manages and ends calls over the internet. SIP trunking is the service that uses SIP to deliver phone "trunks" — the connection between your phone system and the network — over the internet. In short, SIP is the protocol, and SIP trunking is the service built on it.
What is a trunk in telephony?
A trunk is the connection that carries calls between your phone system and the wider telephone network. Traditionally trunks were physical bundles of lines, such as ISDN, sized to the number of simultaneous calls you needed. A SIP trunk does the same job virtually, over your internet connection, with capacity measured in channels rather than physical lines.
What is the difference between SIP trunking and ISDN?
ISDN delivered phone trunks over dedicated physical lines, ordered in fixed blocks. SIP trunking delivers them over the internet and scales in single channels. SIP is generally cheaper, faster to provision and more flexible, which is why it has become the standard replacement for ISDN as that network is switched off in Australia.
Is SIP trunking the same as a cloud phone system?
No. SIP trunking keeps your existing phone system and only replaces the connection to the network. A hosted PBX or cloud contact centre replaces the phone system itself with a vendor-hosted service delivered over the internet. SIP trunking suits businesses keeping their current equipment; a cloud system suits those modernising the whole platform.
How many calls can a SIP trunk handle?
Capacity is measured in channels (also called sessions), where each channel carries one simultaneous call, in or out. You buy as many channels as your peak call volume requires and can add or remove them relatively quickly. This is a key advantage over ISDN, which came in fixed blocks and was slow to change.
Is SIP trunking good for contact centres?
Yes, particularly for contact centres that want to keep an existing phone system while modernising the underlying connection. SIP trunking makes it easy to flex channel capacity for seasonal peaks and supports remote agents. For contact centres replacing the whole platform, a cloud contact centre built on VoIP may be a better fit, and the two approaches are sometimes combined.
Where to Next
Summary: SIP Trunking
SIP trunking connects a phone system to the telephone network over the internet, using the SIP protocol to carry calls in place of physical ISDN lines.
It brings together two older terms: SIP, the protocol that sets up internet calls, and trunks, the connections that carry calls to and from the network. A SIP trunk is simply the virtual, internet-based version of a traditional trunk.
Compared with ISDN it's cheaper, faster to provision and scales in single channels; compared with a hosted PBX or cloud contact centre, it keeps your existing phone system and only replaces the connection. It all runs on VoIP.
In Australia it's the standard way to keep an existing phone system connected as ISDN is switched off through the NBN migration. To compare providers, browse the Business Phone Systems section of the ACXPA Supplier Directory.
- About the Author
- Latest Articles
Sean McGinn has over 25 years of experience in contact centres and operations, he has been a General Manager at companies like General Electric and ME Bank and is now a Director at Callo CX, a CX design agency helping organisations become more customer-centric.
