A recent update to the WhatsApp beta app added VOIP calling, but now the latest, to version 2.12.40, has added a much more Windows Phone like user interface.
tests of the voice quality on a high bandwidth connection shows sound quality to be excellent, and I expect the VOIP feature in WhatsApp to be very popular when it eventually comes to non-beta users of the app.
WhatsApp is the world’s largest messaging service with more than 800 million regular users, and the VOIP calling service has already come to iOS and Android over the last 3 months.