Re: Not sure the comparison is valid
There's a perfectly good reason for not using WhatsApp to make calls.
Last time I checked, most contracts have huge or unlimited allowance for phone calls and text messages, so using either is likely to be free. Many contracts have a fairly modest data allowance, and that's likely to be slurped by things like streaming media and transmission of images. WhatsApp activity comes out of the data allowance, so unless your phone has a WiFi connection it could end up costing you money.