The recent annoucement of WhatsApp Calling got me thinking. If you are receiving a VOIP call you must pay for the data. In other words, unless you’re using an uncapped or free connection, VOIP means both caller and called-partypays, unlike traditional voice for which only the caller pays.
The average data required for a voice call is about 0,5MB/minute and (in South Africa) prepaid data rates are about 10c/MB (USD). Like most emerging markets, South Africans do not have any options for uncapped mobile data.
At one point, you had to pay to receive text messages or calls on a mobile in the USA (thankfully something that never came to Europe) but gladly has disappeared. In time, the issue described above will disappear also - in the Western World, it's largely a non-issue already.