It seems WhatsApp’s latest private Beta brings VOIP calling features. Screenshots posted on Twitter reveal the feature and shows it in action. The Beta version in question is 2.12.38.
The language in the screenshots is not English but still we can make out “mute”, “message” or “voice” options. It is not clear at this moment though when one can expect the feature to arrive with the stable version.
— Emanuele (@mancae90) May 29, 2015