SIP Response Classes

SIP response codes grouped like HTTP — 1xx (Provisional: processing or ringing), 2xx (Successful: request accepted), 3xx (Redirection: try alternate address), 4xx (Client Error: bad request), 5xx (Server Error: server failed), and 6xx (Global Failure: rejected everywhere). SIP-specific codes include 180 Ringing, 183 Session Progress, 407 Proxy Authentication Required, 480 Temporarily Unavailable, 486 Busy Here, and 603 Decline. Final responses (2xx–6xx) terminate a transaction.