I can answer this question, because I used to work at Polycom and this was a common question. The HDX (or any Polycom product) is limited to NAT ON or NAT OFF. There is NO intelligence in the box to assume "Oh, this call is internal, but I see that NAT is enabled, let me just use my internal IP" ON means NAT is ON, whether internal or external address is being used. To get around this, you NEED to put your Polycom, into a DMZ Zone off the FW, allow it to have a public IP address. Your FW rules would allow from Internet to DMZ, or Public IP to public IP> Your internal users would now use the Public IP of the Polycom codec. No more calling via the internal IP address. That is just how it works....
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