They most probably use WebRTC for video and voice calling, and you can't have an unencrypted call in WebRTC.
Since you seem to know much more about that than me, forgive me asking a direct follow-up question:
Is it, if it's WebRTC, only in-transit encryption or would that mean that the calls content isn't visible for WhatsApp/Facebook at all?
WebRTC being peer-to-peer, calls are encrypted end-to-end between peers. So if WhatsApp calls are peer-to-peer, then the call content would not be visible to WhatsApp/Facebook.
However, it's still possible to put a server between two participants so that instead of communicating directly, they would be communicating through this intermediate endpoint which would have access to call content - without call participants even knowing. So ultimately it's a question of trusting a particular service, not the protocol used.
It is end to end encrypted.
Re:Whatsapp: Video Calling
by thewhitetulip on Tuesday November 15, 2016 @04:28AM
(#12956726)
you really believe them?
I do believe, as I wrote, that they promoted end to end encryption. This is also what is directly observable in their statements.
They promoted their end to end encryption promise for messages, so I think they would've promoted it again if it still were true for the video call feature.
edit:
Google, for example, promised their video calls to be encrypted with Duo. https://blog.google/products/duo/meet-google-duo-simple-1-to...