One of the previous posts for a bit more background on WiFi calling ...

http://www.traveltalkonline.com/ubbthreads.php/topics/155902/1.html

All one needs to do is turn on wifi calling at home, like right now, and try it.

As far as charges from your carrier for doing this, we've never had any charges regardless of who we call. Since you're not using a cell phone tower or telecom systems, I'm not sure how the data can even be captured to bill you.