Which SIP response code should be used for maintenance mode

A SIP response code for general maintenance purposes seems not to exist (reference: http://en.wikipedia.org/wiki/开发者_Go百科List_of_SIP_response_codes).

What kind of response code ( 1xx/2xx/3xx/4xx or 5xx ) should be used for maintenance mode?

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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