Whatsapp Free Voice Calling is already available (Android first, Windows and iOS will be available soon) and allows IP calls. This new feature will probably increase the number of times you phone, but there is a factor to consider. In case anyone had not raised: data consumption goes for both the sender and the receiver of the call. This may seem obvious, it is not, because almost everyone call when you have Wi-Fi available, but the callee is probably 3 or 4G.
Whatsapp Free Voice Calling and hidden costs
However, data consumption is quite low: A five minute call is 0.75 MB. But it is worth have it in mind. There are some web that made the numbers:
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This article is also available in Spanish in http://www.archerphoto.eu/costes-y-llamadas-de-whatsapp/