[Serusers] Outbound proxy

Hi all! I'm trying to configure ser to work with all clients behind a nat. I know fwd use ser + asterisk. I noticed that if I have a client behind a nat in fwd I have to set an "Outbound Proxy" (in Xlite for example). The FWD Outbound Proxy is fwdnat.pulver.com:5082. I noticed that there is a "ser" at fwdnat.pulver.com:5082.

Why fwd use an outbound proxy and another "ser" at port 5082 instead using default ser at port 5060? I have to configure 2 distinct ser to solve nat problem or can I use unique ser at port 5060 with nathelper? Using another "ser" as outbound proxy at another port is better? How can I configure ser to be an "outbound proxy"?

Thanks you, Aldo.