For sometime, cellular carriers have made available Femtocell devices to improve calling service coverage in some poor reception areas and also help to decongest their crowded cellular network. Femtocell is a small device often called mini tower. It is normally connected to a home Internet router to reroute cell signal to IP network.
Sprint just announced plan to bypass those devices via a software update to selected phones. Initially, the software will allow owners of the Samsung Galaxy S4 Mini and the Samsung Galaxy S4 Mega phones to place calls via WiFi when WiFi network is available. Call reception and quality over WiFi should be improved drastically when compared to cell network. Sprint plans to bring the same capability to additional devices in 2014.
The capability basically makes your cell phone acting as an IP phone. Calls placed over WiFi will not count toward the calling minute plan and Calls placed from overseas to local numbers are considered local calls.
Note that ongoing call will be disruptive when switching from WiFi to cellular network.