 <p>SIP response codes are splitted in 6 classes</p> <ul> <li><p><code>1xx</code>: <code>Provisional</code> — request received, continuing to process the request; Provisional responses, also known as informational responses, indicate that the server contacted is performing some further action and does not yet have a definitive response. A server sends a 1xx response if it expects to take more than 200 ms to obtain a final response. Note that 1xx responses are not transmitted reliably. They never cause the client to send an ACK. Provisional (1xx) responses MAY contain message bodies, including session descriptions.</p></li> <li><p><code>2xx</code>: <code>Success</code> — the action was successfully received, understood, and accepted;</p></li> <li><p><code>3xx</code>: <code>Redirection</code> — further action needs to be taken in order to complete the request;</p></li> <li><p><code>4xx</code>: <code>Client Error</code> — the request contains bad syntax or cannot be fulfilled at this server;</p></li> <li><p><code>5xx</code>: <code>Server Error</code> — the server failed to fulfill an apparently valid request;</p></li> <li><p><code>6xx</code>: <code>Global Failure</code> — the request cannot be fulfilled at any server.</p> <p>Here you can find <a href="http://www.pjsip.org/pjsip/docs/html/group__PJSIP__MSG__LINE.htm" rel="nofollow">PJSIP struct which holds these codes</a> and <a href="https://en.wikipedia.org/wiki/List_of_SIP_response_codes" rel="nofollow">SIP codes description</a></p></li> </ul>