Cisco TelePresence Administrator's Manual page 210

Dial plan and call processing
About URI dialing
In addition, the DNS records for incoming calls should be configured with the address of the VCS
Expressway as the authoritative gatekeeper/proxy for the enterprise (the DNS configuration examples
[p.383] section for more information). This ensures that incoming calls placed using URI dialing enter the
enterprise through the VCS Expressway, allowing successful traversal of the firewall.
Cisco VCS Administrator Guide (X8.1.1) Page 210 of 507