In general, you can use H323 to call VMRs or endpoints like calling email@example.com with valid DNS settings like putting Google public DNS IP: 22.214.171.124 and 126.96.36.199 .
VC-Connect Live Service Status
How do I make a video call?
Local Country Telephone Dialing Numbers
What firewall changes do I need to make to use this service?
How do I test my video is working?
How to solve Polycom HDX Endpoint Packet Loss
Supported Endpoints and Interoperability
Audio and Video Specifications
Endpoint registering to the wrong location