asterisk users - Nov 2004 - iax busy / unavailable - not registered

hello,

i need some suggestion how to indicate caller that calling number is 
unavailable if some iax user is not registered:

this is what I got in asterisk console:

app_dial.c:727 dial_exec: Unable to create channel of type 'IAX2'
  == Everyone is busy/congested at this time

why is send  to busy congested but user is just unavailable !?
any way to play unavailable message if user  is not registered?

thank you,
Tomaz