Voice missed and outgoing ip

hi everybody…
i’m trying to use asterisk but i have some problems…
communication between two clients in different networks is almost set up but i have still some problems… on one front i can’t hear voice but on the other all is well…
can someone say to me what to check?
in addition, when i filter traffic outgoing from the firewall of my lan, i see it going out with firewall public ip instead astersik server public ip…
where i get wrong?
which kind of nat have i to use?
thks in advance…
best regards…
mike