asterisk users - Jan 2004 - Long time to detect that an IAX2 user is not logged on

I notice that when my dial plan directs a call to an IAX user on my
own asterisk server that is not logged in, it waits the full dial time
before skipping to voice mail. Why is that and is there any way to get it
to skip straight away if the person is not logged in?
 
  - Kim