title: Which SIP response code should be used for maintenance mode
body: <p>A SIP response code for general maintenance purposes seems not to exist (<em>reference:</em> <a href="http://en.wikipedia.org/wiki/List_of_SIP_response_codes" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/List_of_SIP_response_codes</a>).</p>
  <p>What kind of <strong>response code</strong> ( 1xx/2xx/3xx/4xx or 5xx ) should be used for <strong>maintenance mode</strong>?</p>