The latest beta for Android (v2.16.80) now shows the option for video calls, although the functionality is still not active.
Most likely, the feature will be activated server-side in the next update, or the feature can be rolled out in waves like past features have been including voice calling.
Some lucky users were actually able to complete a call, although it is unclear how. Here is one example:
Written by: Andre Yoskowitz @ 15 May 2016 17:37
