1xx—Informational Responses
100 Trying
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

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

http://en.wikipedia.org/wiki/List_of_SIP_response_codes