You just call any destination you want and if:
- Your provider supports video
- The other side's provider supports video
- The other side's recipient supports video
- You have Supports Re-Invite* on your trunk
- You have not disallowed video calls in your trunk
..then the call will be established, and you will have to click the "video" icon on your phone during the call
* = this option may cause other unwanted behavior if your provider does not fully support it.
For now, just call one of your extensions that is logged into the webclient so you can test it.