At the end, the code of the calls was within the latest updates of WhatsApp for Android, and to activate it on your device, you just need a call from someone who already has activated it. You don’t need a rooted device, just the last version of WhatsApp, either in the stable version or in the beta version. Only for Android.
We have been able to try it thanks to a call from Arnold and although mysteriously, the new interface have not been activated to us, we have experienced the expected voice calls, which did not have the expected quality in 4G, surely we have to wait to try them on other networks to speak more accurately about their quality.
It seems that WhatsApp had planned to activate a limited number of Voice Calls, a limit that was exceeded very quickly because, a few hours to allow activate Voice Calls on WhatsApp for Android, it’s no longer allowed more activations.
Of course, the users who had activated the voice calls before still have it activated and can call to other users, but users that received the calls after can not activate it.