Asterisk and NAT


After reading some documentation, there still is something that I am unsure of :

Let’s say I have a NAT’ed network, with a gateway, on which Asterisk is setup.

The NAT’ed boxes REGISTER their sip address to the Asterisk box, so that clients outside the network can contact them.
It looks like the SIP PROXY function deals with this. However, is the Voice connection also proxied ?

If voice is not proxied, then only the SIP messages are relayed, right ? In this scenario, is there a way to actually get NAT’ed machine to be VOICED and VOICE machine outside the NAT ?

Thanks for your help,
Sami Dalouche