asterisk users - Jun 2004 - Busy when not registered

If I try to dial a SIP extension which is not connected/registered with *, I
end up getting a busy indication and the call goes through to the
"busy"
voicemail message.  The extension is listed in the sip.conf, but it is not
connected at the time.
Shouldn't it go to the "unavailable" voicemail message?

Simon Brown