asterisk users - Feb 2005 - Outbound proxy

Hi i need to register with my pstn server but this use outbound proxy, i
have been reading the wiki but not figure how configure de outbound proxy
only say the sentences is this ...

register => user:pass:fromuser@server:port/#

but where specific the outbound proxy ? my isp provider WWTelco don't work
if i not specific this parameter.

Thanks a lot, and sory for my english
George