What is Wi-Fi calling and how does it work?

Other Apple Chat

Before start I would like to do a small annotation
AnonyMouse as always made excellent podcast about
mention that only T/Mobile and At&T were the only carriers in US who support this feature Sprint offer this service as well :)
Does Wi-Fi Calling is the same as FaceTime?
What is Wi-Fi calling?
Instead of using your carrier's network connection, users can make voice calls through a Wi-Fi network. That can include a Wi-Fi connection you have set up at home, or whatever Wi-Fi hotspot you happen to be on when you're out and about, such as a cafe or library. In most ways, it's like any other normal call, and you still use regular phone numbers.
Wi-Fi Calling is now available for iPhone. With Wi-Fi Calling you can experience improved voice service in locations that previously had limited or no mobile network coverage.
Wi-Fi Calling benefits
Enhanced coverage in buildings and areas with challenging network
To activate this feature go to:
Wi-Fi Calling

US Carriers who supportWi-Fi calling up to date
T/Mobile was the first carrier who launched it
Since iOS8
iPhone devices/models who support it from T/M 5S, 5C, 6 and 6 Plus
This feature was launched in iOS8.3 released in April
Sprint’s iPhone who supporting
iPhone 5C, 5S, 6 and 6 Plus
Just joined recently after iOS9
The devices who support according to apple apply to 5C, 5S, 6 and 6 Plus
according to apple website
you can find all carriers all over the world who support Wi/Fi Calling.
Wi/Fi quality
The voice quality in my experience is crystal and very hight quality similar to Skype I could say even better, to have better experience I suggest be close to the stronger Wi/Fi signal

The bad as nothing is perfect I had the following drawback.
I am Mac user and I have enable
Text message forwarding
FaceTime On
to iOS8 allow calls on other devices On under FaceTime
to iOS9 allow calls on other devices under Phone
I was no longer able to receive calls through the Mac
I was able to make calls from the Mac but never came through, I needed go at apple genius and the Mac is excellent working
the only solution to fix it to calls came through the Mac was restoring the iPhone to previous backup and not longer activate Wi/Fi calling
it happens using an iPhone 5S running iOS8.3 or 8.4
If you are one of the users who wanna try this service the best luck
New iPhones 6S and 6S Plus might include Wi/Fi calling as well.