Good day.I have used IPTabels 1.2.5-3,and dont know that it have or not H323 modul .But for recive Call from outside(internet) to inside(throught NAT) - Netmeeting(ATA 186) i use thet rules: 

$IPTABLES -t nat -A POSTROUTING -o $EXTIF -s 192.168.0.0/24 -j MASQUERADE 

  (intern LAN)
PORTFWIP="192.168.0.201" ( PC with Netmeting or ATA 186)

$IPTABLES -A FORWARD -i $EXTIF -o $INTIF -p tcp --dport 1720 -m state --state
NEW,ESTABLISHED,RELATED -j ACCEPT
$IPTABLES -A PREROUTING -t nat -p tcp -d $EXTIP --dport 1720 -j DNAT --to $PORTFWIP:1720
But i want to use NAT without  FORWARD ,PREROUTING with H323 ability for any IP in LAN.Where i can find IPtables with ability of H323(version) or where is the patch(modul) for that ability? Thank your. 

Buy,

Eugen