All calls and texts are free on WhatsApp. It doesn't matter what network.
When on Wi-Fi.
You can also make calls via Facebook for free on Wi-Fi.
Doesen't the receiving party also have to have WhatsApp? or is that for off wifi calls....and if calling the USA (or USA cell phones anywhere) I just use wifi calling at no cost but calling an SXM establishment would have a small charge DEFINITELY not $16/day