Calling requires software update but no monthly fee; only supported in the U.S.
T-Mobile may soon no longer be the only major carrier offering first-party Wifi calling, as Sprint apparently is preparing to launch a system of its own. According to Sprint training materials we've received, Sprint Wifi Calling would be enabled on at least two of its current Android handsets through maintenance updates and on new devices running Android 4.2 and above.
We've reached out to Sprint for any sort of confirmation and will update if and when we hear back.
In the meantime, let's look at more details of Sprint's anticipated Wifi Calling system.
As you'd expect, calls made over Wifi will not count against your voice minutes, and are in no way interchangeable with calls made over CDMA — i.e. calls can't be handed off between the two systems.
In order to take advantage of Wifi Calling, a customer will simply need a compatible device, enable the feature through a web interface, http://sprint.com/manage, and begin using it. No monthly charge will be assessed for turning on Wifi Calling. The training materials indicate that there will simply be a Wifi Calling toggle in the settings of "approved" Sprint Android devices going forward.
An interesting wrinkle here is that you must have Android Location Services turned on in order to make Wifi calls, and that it will only be a supported feature within the U.S., U.S. Virigin Islands and Puerto Rico. Sprint explicitly states that Wifi Calling will not be approved or supported from other locations or while using a VPN. Outbound calls over Wifi won't be allowed without a CDMA signal present either — Sprint claims this is for 911 reasons.
A timeline for the availability of Wifi Calling on Sprint devices isn't yet known, but all documentation currently available indicates that a launch is imminent, at least on the Samsung Galaxy S4 Mini and Galaxy Mega.
So hang tight, Sprint fans. Wifi calling is coming. Probably.