Solution for Clients Behind NAT?

Hi, i rent a Dedicated server in USA for route my outbound calls but i’am behind NAT.
What solution are possible for free or cheap to fix this?
NOTE:* Vonage and that ITSP’s have that issue fixed, so, how can i make it work properly, because at my office, sometimes, the calls do pass succesfully.

I’m open to get suggestions!

Here is some information on configuring Asterisk for NAT traversal:

The config entries that are relevant for this function are nat=yes and / or qualify=yes.

Thank you.

Okey, but exist some source code, an aplicattion to install that make the RTP proxy server, do you know a RTP Proxy server services provider??
How is the easy way to do it?