21.6 Global Failures 6xx

   6xx responses indicate that a server has definitive information about
   a particular user, not just the particular instance indicated in the
   Request-URI.

21.6.1 600 Busy Everywhere

   The callee's end system was contacted successfully but the callee is
   busy and does not wish to take the call at this time.  The response
   MAY indicate a better time to call in the Retry-After header field.
   If the callee does not wish to reveal the reason for declining the
   call, the callee uses status code 603 (Decline) instead.  This status
   response is returned only if the client knows that no other end point
   (such as a voice mail system) will answer the request.  Otherwise,
   486 (Busy Here) should be returned.

21.6.2 603 Decline

   The callee's machine was successfully contacted but the user
   explicitly does not wish to or cannot participate.  The response MAY
   indicate a better time to call in the Retry-After header field.  This
   status response is returned only if the client knows that no other
   end point will answer the request.

21.6.3 604 Does Not Exist Anywhere

   The server has authoritative information that the user indicated in
   the Request-URI does not exist anywhere.

21.6.4 606 Not Acceptable

   The user's agent was contacted successfully but some aspects of the
   session description such as the requested media, bandwidth, or
   addressing style were not acceptable.

   A 606 (Not Acceptable) response means that the user wishes to
   communicate, but cannot adequately support the session described.
   The 606 (Not Acceptable) response MAY contain a list of reasons in a
   Warning header field describing why the session described cannot be
   supported.  Warning reason codes are listed in Section 20.43.

   A message body containing a description of media capabilities MAY be
   present in the response, which is formatted according to the Accept
   header field in the INVITE (or application/sdp if not present), the
   same as a message body in a 200 (OK) response to an OPTIONS request.

   It is hoped that negotiation will not frequently be needed, and when
   a new user is being invited to join an already existing conference,
   negotiation may not be possible.  It is up to the invitation
   initiator to decide whether or not to act on a 606 (Not Acceptable)
   response.

   This status response is returned only if the client knows that no
   other end point will answer the request.

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