SIP和http协议很相似，包括respons code也是基于数字的，sip的respons code是由3位的阿拉伯数字组成，如100， 200等，第一位范围从1到6。下面列出这些respons code的含义，这里的列出的只是部分。
1xx—Informational Responses
- 100 Trying: extended search being performed may take a significant time so a forking proxy must send a 100 Trying response
- 180 Ringing
- 181 Call Is Being Forwarded
- 182 Queued
- 183 Session Progress
2xx—Successful Responses
- 200 OK
- 202 accepted: It Indicates that the request has been understood but actually can’t be processed
- 204 No Notification [RFC5839]
3xx—Redirection Responses
- 300 Multiple Choices
- 301 Moved Permanently
- 302 Moved Temporarily
- 305 Use Proxy
- 380 Alternative Service
4xx—Client Failure Responses
- 400 Bad Request
- 401 Unauthorized (Used only by registrars or
user agents. Proxies should use proxy authorization 407) - 402 Payment Required (Reserved for future use)
- 403 Forbidden
- 404 Not Found (User not found)
- 405 Method Not Allowed
- 406 Not Acceptable
- 407 Proxy Authentication Required
- 408 Request Timeout (Couldn’t find the user in time)
- 409 Conflict
- 410 Gone (The user existed once, but is not available here any more.)
- 412 Conditional Request Failed
- 413 Request Entity Too Large
- 414 Request-URI Too Long
- 415 Unsupported Media Type
- 416 Unsupported URI Scheme
- 417 Unknown Resource-Priority
- 420 Bad Extension (Bad SIP Protocol Extension used, not understood by the server)
- 421 Extension Required
- 422 Session Interval Too Small
- 423 Interval Too Brief
- 424 Bad Location Information
- 428 Use Identity Header
- 429 Provide Referrer Identity
- 433 Anonymity Disallowed
- 436 Bad Identity-Info
- 437 Unsupported Certificate
- 438 Invalid Identity Header
- 480 Temporarily Unavailable
- 481 Call/Transaction Does Not Exist
- 482 Loop Detected
- 483 Too Many Hops
- 484 Address Incomplete
- 485 Ambiguous
- 486 Busy Here
- 487 Request Terminated
- 488 Not Acceptable Here
- 489 Bad Event
- 491 Request Pending
- 493 Undecipherable (Could not decrypt S/MIME body part)
- 494 Security Agreement Required
5xx—Server Failure Responses
- 500 Server Internal Error
- 501 Not Implemented: The SIP request method is not implemented here
- 502 Bad Gateway
- 503 Service Unavailable
- 504 Server Time-out
- 505 Version Not Supported: The server does not support this version of the SIP protocol
- 513 Message Too Large
- 580 Precondition Failure
6xx—Global Failure Responses
- 600 Busy Everywhere
- 603 Decline
- 604 Does Not Exist Anywhere
- 606 Not Acceptable
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