Just days after being purchased by Facebook for a cool $19 billion and saying nothing would change, WhatsApp is promising one change later this year: "free" voice calls . Support for voice is a useful addition to any messaging platform, but remember: it won't be free. Anytime you're using 3G or 4G, you'll have to pay for the data.
This is true of Skype, Facetime, Viber, BBM and any other service you can think of: "free" alternatives to voice or texting will use data connections instead. The amount of data needed for text messages is tiny. The amount for voice still isn't enormous, but it is bigger.
Given that so many Australian phone plans now include free texting and calls but have relatively stingy data allowances, switching to voice calls over a data network can be a false economy. If you have the right prepaid plan, the main advantage will be staying in contact with other users, rather than the cost. It won't matter if you're using Wi-Fi at home or in the office, but outside that environment it's worth thinking carefully.
WhatsApp Is Getting Voice Calling [Gizmodo]