I would appreciate recommendations for IP Phones to be used with FreePBX. We will be using approximately 50 phones with FreePBX behind a router. We need good NAT support on those phones. The FreePBX is installed from the iso image available with AsteriskNOW website.
If I understand your post the phones and the server are on two different networks?
Do your self a favor and setup a VPN you will never get acceptable results with NAT.
I recommend the Aastra 67xx series. They are tightly integrated with FreePBX and have models from a sub $100US displayphone to a color touch screen executive set.