iptables rules for vonage

I have a Cisco ATA 186 that comes with the vonage phone service. I
also have a linux NAT gateway running iptables.

I will configure my dhcp server to always give the cisco the same IP
address; however, what iptables port forwarding do I need to put in so
the cisco will work ?

If I connect the cisco ATA 186 directly to my internet connection, it
works and I can make calls. But once it is behind the linux NAT'ing
box it cannot.

Thanks in advance,

--Rob