Sprint is preparing to become the second major carrier to offer WiFi calling. According to leaked Sprint marketing materials (below), Sprint WiFi calling would be enabled on at least two current Android handsets through maintenance updates and new devices that run on Android 4.2 and above.
Calls made over WiFi would not count towards voice minutes and the calls cannot be handed off between the WiFi connection and CDMA. You must also have Android Location Services turned on for making WiFI calls. This feature would be supported in the U.S., the Virgin Islands, and Puerto Rico.