Session Initiation Protocol (SIP) response codes and timing patterns are used in detecting invalid and no answer numbers. The following table describes common SIP response codes and descriptions:
| SIP Code | Description |
|---|---|
| 200, "OK" | Indicates that the request was successful. |
| 403, "No Headers" | Indicates that the WebRTC call does not contain header information. |
| 403, "Forbidden auth ID" | Indicates that the server has refused to fulfill the request. Authorization does not fix the issue. |
| 403, "No password in memory" | |
| 403, "No Active Session" | Indicates that an active session does not exist. |
| 403, "Preload Route denied" | |
| 404, "No destination" | The server has definitive information that the user does not exist at the domain specified in the request URI. Also, this status is returned if the domain in the request URI does not match any of the domains handled by the recipient of the request. |
| 404, "Not here" | Indicates that the destination you are trying to reach is not reachable. |
| 480, "No routing pool available" | Indicates an internal routing message. |
| 483, "Too Many Hops" | Indicates that the server is rejecting bad calls. |
| 500, "Internal Error" | Indicates that the server could not fulfill the request because of some unexpected condition. |
| 500, "Unable to failure route" | |
| 503, "Service Unavailable" | Indicates that the server is temporarily unable to process the request due to a temporary overloading or maintenance of the server. The server might indicate when the client should retry the request in a Retry-After header field. |
| 599, "No routing rule available for this scenario" |