SIP trunking explained: a guide for non-technical business owners

The old way vs the new way

• Telstra installs physical wires to your building.
• Each line carries one or two calls; need more lines, need more wires.
• Adding/removing lines = installer visit + truck roll + waiting.
• Call costs include the telco’s infrastructure tax.
• If the copper breaks (rodents, roadworks, water damage), you’re down.

New way (SIP trunking):

• Your existing internet connection carries voice.
• One SIP trunk handles many simultaneous calls; scale up or down in software.
• Adding/removing channels = a config change.
• Call costs are wholesale-rate based; cheaper.
• If one internet path fails, SIP can failover to mobile or alternate paths.

Australian ISDN is being decommissioned by 2030; copper PSTN is largely gone. SIP is the only realistic path for modern business telephony in 2026.

What “SIP” actually means

SIP - Session Initiation Protocol - is the standardised set of rules computers use to set up, manage and tear down voice and video calls over the internet. Your phone system sends SIP messages to your SIP provider’s servers; their servers handle the routing onto the public phone network (PSTN).

SIP is open-standard. It’s not Telstra-specific, RingCentral-specific or anyone-specific. That’s the key advantage: SIP trunks work with any compatible phone system, and your phone system can switch SIP providers without changing how it works internally.

Channels (concurrent calls)

SIP trunks are sized in “channels” - the number of simultaneous calls the trunk can handle. A 10-channel SIP trunk can carry 10 calls at once (inbound, outbound or internal).

Sizing rule of thumb for Australian SMBs: most businesses peak at 1 channel per 5–8 users. A 40-user office typically needs an 8-channel SIP trunk. Contact centres need 1:1 channel-to-agent.

Adding channels later is a config change, not a hardware install.

SIP and your phone system

Your phone system (3CX in our world) is the brains; the SIP trunk is the connection to the outside world. SIP carries:

• Inbound calls from external numbers (1300, landline, mobile).
• Outbound calls to external numbers.
• DID routing - direct-inward-dialling, so each extension can have its own external number.

Most modern phone systems support multiple SIP trunks for redundancy - primary and failover with different providers.

Pricing models

Two components in SIP trunking costs:

Per-channel monthly fee. Typically AUD $5–15 per channel per month for Australian SIP providers. An 8-channel trunk is $40–120/month in fixed cost.

Per-minute call charges. Local Australian landline: $0.02–0.10/min. Australian mobile: $0.10–0.25/min. International rates vary by destination. Inbound on standard numbers is usually free; 1300/1800 inbound has receive charges.

CPS bundles SIP into the monthly 3CX managed-service cost - you don’t manage a separate trunk invoice. The bundle covers a generous Australian local/national/mobile allowance with flat overage rates.

Choosing an Australian SIP provider

If you’re not bundling, what to look for:

• Australian-based with redundant gateways - Sydney + Melbourne datacenters with diverse network paths.
• Number portability - must accept ports from Telstra, Optus, TPG and other Australian carriers.
• Codec support - G.711 (standard), G.729 (compressed), Opus (modern wideband). 3CX supports all.
• Call recording compatibility - your phone system records, the trunk just carries audio; should be transparent.
• Emergency calls (Triple Zero / 000) - must support Australian emergency calling with correct location info.
• Failover - automatic switch to backup gateway if primary fails.
• Pricing transparency - flat per-channel fee + transparent per-minute rates. Watch for “bundled minutes” plans that look cheap then bite on overage.

Most major Australian SIP providers (MNF Group, Symbio, Aussie Broadband, Vonex, several others) meet these criteria. CPS uses bundled Australian SIP infrastructure (Cloudflare-protected) but supports any compatible SIP trunk if you have an existing relationship.

Quality and reliability

SIP call quality depends on three things:

Internet bandwidth. ~50 Kbps per call (G.711) or ~30 Kbps (G.729). An 8-channel trunk uses up to 400 Kbps peak. Most NBN 50/20+ connections handle this comfortably alongside normal office traffic.

Network quality (jitter, packet loss). More important than raw bandwidth. Spikes in jitter cause audio dropouts. Most Australian NBN connections are stable enough; satellite and 4G LTE backup connections sometimes have jitter issues during congestion.

Quality of Service (QoS). Prioritise voice traffic on your router/firewall over other traffic. Standard config on business-grade routers; we provide templates.

In practice: Australian business broadband + a properly-configured router = call quality indistinguishable from old copper.

Disaster recovery and failover

SIP enables disaster recovery patterns that copper couldn’t:

• Failover trunks - second SIP provider takes over if primary fails.
• Mobile failover - calls forward to mobile numbers if both internet paths fail.
• Cloud-hosted PBX - if your office burns down, the PBX keeps working in the data centre and staff dial in from mobile/web.
• Number redundancy - your numbers live with the SIP provider, not at your office.

For business-continuity-critical applications (medical reception, contact centres handling outages, financial services), we configure multi-trunk failover as standard.

SIP and 3CX

3CX is SIP-native. It speaks SIP to your trunk provider, and to your handsets and softphones. The combination gives you:

• Bundled Australian SIP trunking (no lock-in)
• Per-system pricing on the PBX (not per user)
• Auto-configuration of phones via SIP provisioning
• Australian calling bundled (if you’re with CPS)

More on 3CX hosting and SIP →

Common questions