Glossary
SIP
The open-standard signalling protocol used by modern phone systems to set up, manage and tear down voice and video calls over the internet.
also known as: Session Initiation Protocol
SIP (Session Initiation Protocol) is the open-standard signalling protocol that modern phone systems use to set up, manage and tear down voice and video calls over the internet. It’s the foundation of VoIP (Voice over Internet Protocol) and cloud PBXs including 3CX.
What SIP does
SIP handles the signalling layer of a call:
- Setup - “I want to call this number” → finding the destination, negotiating capabilities, establishing the connection.
- Modification - transfer, hold, conference, codec re-negotiation.
- Teardown - hang up cleanly on both ends.
SIP itself does not carry the actual audio. That’s the job of RTP (Real-time Transport Protocol). SIP sets up the call; RTP carries the conversation.
Why SIP matters
SIP is open-standard. It’s not proprietary to any vendor. That means:
- Any SIP-compatible phone system can talk to any SIP-compatible trunk provider.
- You can change SIP provider without changing your phone system.
- You can change your phone system without changing your SIP trunk.
- Handsets from many manufacturers (Yealink, Fanvil, Snom, Polycom, Cisco) all speak SIP and work with any SIP PBX.
That openness is why 3CX deliberately built on SIP - and why per-user SaaS PBXs that lock you to proprietary trunks are increasingly looking like the old way of doing things.
SIP in business phone systems
A typical Australian business phone system using SIP:
- Phone system (3CX) - speaks SIP to handsets and trunk providers.
- Handsets and softphones - speak SIP to the phone system.
- SIP trunk provider - connects the phone system to the public phone network (PSTN).
The PBX is the brains; SIP is the language; the SIP trunk is the connection to the outside world.
SIP and security
Modern SIP uses SIP over TLS (encrypted signalling) and SRTP (encrypted media). All 3CX deployments use these by default. Unencrypted SIP (UDP/5060) is still common in legacy environments but should be avoided for business use.
See also
- [[sip-trunking]] - virtual phone lines using SIP
- [[hosted-pbx]] - phone systems running in data centres that use SIP
- [[cloud-pbx]] - cloud-based PBXs (almost all SIP-based)
- [[rtp]] - the protocol that carries the actual audio
