VoIP glossary · Session Initiation Protocol
What is SIP?
SIP (Session Initiation Protocol) is the signalling protocol that sets up, changes and ends VoIP calls, using text messages similar in style to HTTP.
SIP explained
A call starts with an INVITE from the caller. The other side responds with provisional replies like 100 Trying and 180 Ringing, then 200 OK when answered. The caller confirms with ACK, audio flows over RTP, and either side ends the call with BYE. Failures return error codes such as 403, 404 or 503.
SIP usually runs on port 5060 over UDP or TCP, or 5061 over TLS. Reading a SIP trace and knowing what each response means is the single most useful skill for troubleshooting a trunk.
Example
INVITE → 100 Trying → 183 Session Progress → 200 OK → ACK → (conversation) → BYE → 200 OK.
Put it into practice
Prepaid SIP trunks and wholesale routes from $200. Ask sales for rates or the NOC for a technical answer.