asterisk users - Apr 2004 - SIP response 404 "Not Found" AND circuit-busy ??

I have a dlink dvg-1120s voip-router. I can make calls out from the router,
but when calling the router I got

   -- Executing Dial("SIP/2010-b437", "SIP/2021|30|r") in
new stack
    -- Called 2021
    -- Got SIP response 404 "Not Found" back from 62.79.78.74
    -- SIP/2021-473b is circuit-busy


What does this meen ? Or what can I do ?
The router is behind nat, but if I put the router on the same network as
asterisk it work ok

/Hans-Henrik Andresen