Communicating two client registred in two different gateway


I have registered two clients within two different servers which pings each other. The dial plan I put in extensions.conf is very simple:

exten => 600,1,Dial(SIP/ab@fedora)

but when I lauch an ethereal whith a sip filter, I notice that fedora respond the other server(where the dialer is registered) with:
404: not found, although that ab declare that it is registered.
please help.