What Is SIP? Session Initiation Protocol Explained
What Is SIP?
SIP (Session Initiation Protocol) is the signalling protocol that makes VoIP calls possible. It handles the setup, management and termination of voice and video sessions over the internet. Think of SIP as the technology that dials the number, rings the phone and connects the call.
How SIP Works
- INVITE — your phone sends a SIP INVITE message to initiate the call.
- Ringing — the receiving phone rings and sends back a provisional response.
- Answer — when the call is answered, a final response is sent and the media (audio) stream begins.
- BYE — when either party hangs up, a BYE message ends the session.
SIP Trunking
SIP trunking is the virtual equivalent of a traditional phone line. Instead of physical copper wires connecting your phone system to the telephone network, SIP trunks use your internet connection. This eliminates line rental costs and allows unlimited concurrent calls limited only by your bandwidth.
SIP vs Traditional Phone Lines
| Feature | Traditional (ISDN/PSTN) | SIP |
|---|---|---|
| Connection | Copper phone lines | Internet broadband |
| Cost per line | £15–25/month | £3–10/month |
| Scalability | Install new physical lines | Add channels instantly |
| Features | Basic | Full VoIP feature set |
| PSTN switch-off | Being discontinued 2027 | Future-proof |
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