The WI-FI calling is quite a new feature from carriers. T-Mobile was the first one that had this feature for it's customers. Now it's Sprint turn to introduce it. A leaked document shows that tomorrow WI-FI calling feature will be available on Samsung Galaxy S4 and Galaxy Mega from Sprint. These are the first devices which receive this feature, but it's likely that in the next weeks other devices will receive the update too. These update will be delivered through a OTA update which will come on the handsets. The interesting part is that beside voice calls, you could send text messages through Wi-FI. All these service, voice calls and text messages won't count to your bill. The document says that this feature will be turned on simply through a quick toggle.
The tricky part is that you must register manually every new WI-FI network that you want to use. Next, you must introduce an address for that network to make it accessible. The network's address will be added through Google's features. To use this feature, your device must have CDMA connectivity as the device must know if you're signal is received form that address, and that network is a Sprint authorized one. The feature will be available in U.S.A., U.S. Virgin Islands and Puerto Rico.
So it's time to get a Sprint device and start calling free!