In a nutshell, making calls over Wi-Fi will not eat up the minutes in your voice plan, and are of course, not able to be interchangeable with calls made over CDMA. For example, calls cannot be handed off between CDMA and Wi-Fi. You will need to make sure that you have a compatible device first, followed by enabling this unique feature via a web interface, and you’re good to go.
At the time of publishing, T-Mobile remains the only major mobile carrier that offers first party Wi-Fi calling capability, but from what we have heard today, it seems that this particular pseudo monopoly might eventually come to an end if everything goes down fine with Sprint. As a Sprint user, are you looking forward to Wi-Fi calling capability over your smartphone?