asterisk users - Aug 2006 - 480 "Temporarily Unavailable" message

Hi everybody!


I have a SIP peer correctly registered on my asterisk server (Status: OK 
(2ms)). I can call the peer normally from another peers, os th DND is no 
set. But sometimes I got

-- Got SIP response 480 "Temporarily Unavailable" back from
172.16.34.17
-- SIP/XXX-d910 is circuit-busy

The peer never loses its registry and there are no packet losses between 
it and the server.

Can someone help me debug and resolve this problem?

Thanks a lot.