Telecoms glossary • S
SIP (Session Initiation Protocol)
The protocol that sets up, manages and ends every VoIP call. If your phone rings, SIP made it ring.
What is SIP?in VoIP and business telephony
1. The signalling protocol that sets up, manages and terminates VoIP calls – ringing the far end, agreeing a codec and closing the session when someone hangs up.
2. SIP never carries the voice audio itself. That job belongs to RTP, which runs alongside it. SIP is the control layer; RTP is the media layer.
How a SIP call actually happens
Dialling a number starts a short exchange of text messages between your phone and your provider. The whole sequence completes in milliseconds.
Your handset sends a SIP INVITE. The provider locates the destination and forwards the invitation. When the far end accepts, both sides negotiate a codec, port numbers and a media path. Only then does RTP begin carrying audio. A SIP BYE ends it.
A SIP call, message by message
That split is the most useful thing to know about SIP. One-way audio means SIP succeeded and RTP failed – nearly always a NAT or firewall problem at your end, not a provider fault.
Why SIP matters commercially
SIP is a published standard, and that is the whole commercial point. A handset from one manufacturer registers against a Cloud PBX from a second provider over a line from a third.
In practice you can change provider without replacing handsets, and change handsets without changing provider. A vendor claiming otherwise is describing their own lock-in, not a limit of the technology. It is also why dozens of South African providers can compete on price: they are all speaking the same protocol to the same hardware.
SIP is not the same as SIP trunking
- SIP
- The protocol. Free, published, implemented by everyone.
- SIP trunking
- A commercial product. Voice channels sold to you, connecting an on-premise PBX to the public phone network. It uses SIP; it is not SIP.
The distinction matters at quoting time. Trunks are priced per concurrent channel, not per user, so a 40-person office that never runs more than six simultaneous calls buys six channels – not forty.
Ask before you sign
Ask whether the platform is standards-compliant SIP or a modified stack. If handsets must be bought from the provider, price the lock-in in.
Ask what happens to your handsets if you leave. On an open SIP platform they re-register elsewhere with a config change, not a skip full of hardware.
About this entry
Definitions are written for South African business buyers and checked against primary sources – provider documentation, standards bodies and ICASA – not vendor marketing. Reviews are independent and sponsors are always disclosed. Read our editorial policy and scoring methodology.
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