What Is SIP Trunking?

SIP trunking is the modern, internet-based way of connecting a business phone system to the public phone network. It replaces the physical copper or ISDN lines that used to run into your building with a single internet connection that can carry many simultaneous calls. For South African businesses, it’s the natural replacement as Telkom decommissions its copper network and ISDN is retired.

How SIP trunking works

“SIP” stands for Session Initiation Protocol — the technology that sets up and manages voice calls over the internet. A “trunk” is the connection that carries those calls to and from the public phone network. A SIP trunk gives your PBX or cloud PBX one internet-based connection capable of multiple concurrent calls — called “channels” — instead of multiple physical lines.

You choose how many channels you need (roughly, how many calls you expect at once), and you can scale that up or down in software without anyone running new cables.

Why businesses use SIP trunking

• Lower call costs than legacy line rates, especially for mobile and national calls.
• No physical line rental — one internet connection replaces multiple lines.
• Flexible capacity — add or remove channels easily.
• Disaster resilience — calls can reroute to mobiles or another site automatically.
• Future-proof — copper and ISDN are being retired; SIP is the modern standard.

SIP trunking vs cloud PBX

These are related but different. A SIP trunk is the connection between your phone system and the phone network. A cloud PBX is the whole phone system. If you already have a capable on-site PBX, SIP trunking can modernise its connectivity without replacing it. If you’re starting fresh, a cloud PBX (which includes SIP connectivity) is usually simpler. (See SIP Trunking Explained for the deeper guide.)

What it costs

SIP trunking typically combines a small monthly per-channel fee with low per-minute rates — in the SA market roughly 24–30c/min to landlines and around 79c/min to mobiles, often bundled. Most providers don’t charge setup fees. Most businesses see a meaningful reduction in their phone bill compared with legacy lines.

What you need to use SIP trunking

• A SIP-compatible PBX (almost all modern on-site and cloud PBXs qualify).
• A reliable internet connection with enough bandwidth for your concurrent calls.
• An ICASA-licensed SIP provider with quality routing and local support.

Why this matters in South Africa now

Telkom is decommissioning its copper network and ISDN is being phased out. Businesses still relying on those legacy lines face rising costs and an eventual hard stop. SIP trunking is the natural replacement — modern, flexible, and not tied to ageing infrastructure. (See SIP Trunking vs ISDN.)

Frequently asked questions

Is SIP trunking the same as VoIP?

SIP is the most common protocol used to deliver VoIP calls. VoIP is the broad concept; SIP trunking is the specific way businesses connect their phone system to the network.

Can I keep my existing business numbers?

Yes — your numbers port to SIP, so customers reach you exactly as before.

How many channels do I need?

Roughly the maximum number of calls you expect at the same time. A good provider sizes it from your call patterns, and you can adjust later.

Will call quality match a landline?

On a decent connection with a quality provider, yes — often clearer.

Modernise your business lines with SIP

NexysCloud provides ICASA-licensed SIP trunking and cloud PBX built for South African conditions.

Book a consultation or call 010 110 7100.