A SIP response code for general maintenance purposes seems not to exist (reference: http://en.wikipedia.org/wiki/List_of_SIP_response_codes).
What kind of response code ( 1xx/2xx/3xx/4xx or 5xx ) should be used for maintenance mode?
503 Service Unavailable, see RFC3261, chapter 21.5.4 (Link). BTW, you should retag the question, although the error code scheme is similiar, SIP and HTTP are different protocols