Calls made over Wi-Fi won't count against your minutes, but they cannot be handed off to a CDMA network. That means that once you leave the range of a Wi-Fi signal, your call is dropped faster than a smoking hot biscuit coming out of the oven.
Setting up the service would be a snap.You would need one of the phones supporting calls over Wi-Fi and activate the service through Sprint's online account management page. And then...you use it. Enabled phones will have a Sprint Wi-Fi calling toggle under "Settings" in the future.
There is a caveat that the more paranoid amongst us will probably object to. To use the Wi-Fi calling feature for Sprint, the phone's Android Location Services will have to be turned on. The service also will only work in areas where a CDMA signal exists. Sprint says that this is due to 911 calling capabilities.