I am experiencing exactly the same problem described here
Polycom cannot see or hear the other VC, and the other(s) VC can see and hear Polycom.
I have checked firewalls and settings, even unchecked the checkbox "NAT is H323 Capable, but no way.
I captured the packets and I could find out the the other VC is sending the UDP packets to private IP instead of public IP, so the packets don't reach the destination.
I tried with different options in the NAT (Auto, Manual, etc) and everything the same, not working...
Thank yiu very much in advance