Response received                     Cause value in the REL
   -----------------                     ----------------------
   400 Bad Request                       41 Temporary Failure
   401 Unauthorized                      21 Call rejected (*)

   402 Payment required                  21 Call rejected
   403 Forbidden                         21 Call rejected
   404 Not found                          1 Unallocated number
   405 Method not allowed                63 Service or option

                                            unavailable
   406 Not acceptable                    79 Service/option not
                                            implemented (+)
   407 Proxy authentication required     21 Call rejected (*)

   408 Request timeout                  102 Recovery on timer expiry
   410 Gone                              22 Number changed
                                            (w/o diagnostic)
   413 Request Entity too long          127 Interworking (+)

   414 Request-URI too long             127 Interworking (+)
   415 Unsupported media type            79 Service/option not
                                            implemented (+)
   416 Unsupported URI Scheme           127 Interworking (+)

   420 Bad extension                    127 Interworking (+)
   421 Extension Required               127 Interworking (+)
   423 Interval Too Brief               127 Interworking (+)
   480 Temporarily unavailable           18 No user responding

   481 Call/Transaction Does not Exist   41 Temporary Failure
   482 Loop Detected                     25 Exchange - routing error
   483 Too many hops                     25 Exchange - routing error
   484 Address incomplete                28 Invalid Number Format (+)

   485 Ambiguous                          1 Unallocated number
   486 Busy here                         17 User busy
   487 Request Terminated               --- (no mapping)
   488 Not Acceptable here              --- by Warning header

   500 Server internal error             41 Temporary failure
   501 Not implemented                   79 Not implemented, unspecified
   502 Bad gateway                       38 Network out of order
   503 Service unavailable               41 Temporary failure

   504 Server time-out                  102 Recovery on timer expiry
   504 Version Not Supported            127 Interworking (+)
   513 Message Too Large                127 Interworking (+)
   600 Busy everywhere                   17 User busy

   603 Decline                           21 Call rejected
   604 Does not exist anywhere            1 Unallocated number

Nortel usa RFC 3398 para el mapping (o eso dicen)
   606 Not acceptable                   --- by Warning header