SIP Response Codes: Understanding VoIP Call Flow

SIP Response Code Classes

SIP response codes follow a structure similar to HTTP, with six classes:

1xx — Provisional

Call is in progress but not yet completed.

• 100 Trying — Server received the INVITE, processing
• 180 Ringing — The callee's phone is ringing
• 181 Call Is Being Forwarded — Being redirected to another number
• 183 Session Progress — Early media (ringback tone from callee's network)

2xx — Success

• 200 OK — Call accepted, or transaction completed

3xx — Redirection

• 301 Moved Permanently — User has permanently moved
• 302 Moved Temporarily — Try another address temporarily

4xx — Client Failure

• 401 Unauthorized — Needs authentication (digest auth)
• 403 Forbidden — Server refuses the request
• 404 Not Found — User not found at the domain
• 408 Request Timeout — No timely response
• 480 Temporarily Unavailable — Callee is offline
• 486 Busy Here — Callee is on another call
• 487 Request Terminated — Call cancelled by caller

5xx — Server Failure

• 500 Server Internal Error — SIP server error
• 503 Service Unavailable — Server overloaded

6xx — Global Failure

• 600 Busy Everywhere — All known locations are busy
• 603 Decline — Callee explicitly rejected the call
• 604 Does Not Exist Anywhere — User doesn't exist in the system

Typical Call Flow

Caller → INVITE → Server → 100 Trying → 180 Ringing → 200 OK → ACK

If the callee is busy: 180 Ringing → 486 Busy Here

If the caller hangs up: CANCEL → 200 OK → 487 Request Terminated