Hi,
1: Your site doesn't use any VPN to connect to the central site or to any branch site, right?
2: All other sites use VPNs to connects to the Central site, including he site which is not working.Right?
If above two statement is true then VC communication for your site should always through the Internet, as it can't form the VPN. BTW, do you have any gatekeeper/Server etc at the Central site which helps in setting up the call? Or what is the dependency of the Central site in the VC?
Also, you Can you try below:
1: You are using MIP, mean an IP is dedicated to the VC setup. Can you bypass the firewall and connect the VC setup directly to the public IP and check?
2: I was checking some polycom HDX6000 docs and can see an option where you can mentioned public IP in 1:1 configuration, enable fixed port and Enable H.460 for the ease, but not sure about this.
Thanks,
Vikas