asterisk users - Nov 2004 - Forwarding SIP calls to another SIP Proxy (Peer)

Hey there people, i've been scouring the voip-info.org wiki for a few 
hours and dont seem to be able to do anything in regards to this, so I 
hope someone can help me here.

Now i Have my current configuration as thus..

X-Ten (SIP) -> Asterisk -> Mutliple Peers via SIP or IAX2

Now on one of those SIP Peers id like to call users by their usernames... eg
netadmin@some-isp.net, however I cant find a dialing rule example so 
when it see's the @some-isp.net it forwards this call to the relavant 
SIP Peer and dials that username...

Im gonna be sitting here googlings and reading the voip-info site a bit 
more but anything in the meantime will help me accomplish this task!

Thanks in Advanced if someone can help!

Matthew K
Adelaide,Australia based Asterisk Persong trying to Learn cool stuff :)