IF IT DOES NOT WORK ON YOUR DEVICE, PLEASE DO NOT RATE
It is not supposed to work on every device. If it does not work, please report (with message ) here : yangyz20191101@gmail.com 
What is WiFi calling?
Instead of using your carrier's network connection, you can make voice calls via a Wi-Fi network. That could mean using 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 at a cafe or library. In most ways, it's like any other phone call, and you still use regular phone numbers.
Why would I want that?
WiFi calling is especially useful when you're in an area with weak carrier coverage. For example, when you're traveling to the residential countryside, or you're in a building with spotty reception. You may already be familiar with using Wi-Fi to send messages when SMS texting is unavailable (apps like Kik and Facebook Messenger provide these services) -- and the same applies when you're trying to place a call. With Wi-Fi, you can call a friend up even if you're in a dingy, underground bar (assuming you can connect to the bar's Wi-Fi, that is).
What carriers and phones support this service?
All four major US carriers (T-Mobile, Sprint, AT&T and Verizon) provide built-in Wi-Fi calling. Republic Wireless and Google Project Fi provide Wi-Fi calling on certain phones too. The former carries eight Android phones, while five phones (including the Google Pixel, Pixel XL and Nexus 6P) work on the latter. Republic Wireless gets support from Sprint's network if connection is unavailable, while Google uses T-Mobile, Sprint and US Cellular.
All of T-Mobile phones offer Wi-Fi calling built-in. As for Sprint, Wi-Fi calling is available on a number of iPhone models that run iOS 9.1 or higher. Several Android devices have the service as well, but you'll need to check through your phone's Settings menu to see if you have it. 
Does it cost more?
For domestic calls, it doesn't cost any extra as such. But making calls over Wi-Fi can come out of your regular minutes allowance depending on your carrier and your phone plan. 

Why need the APP?
Today , Many retail mobiles have "Wifi Calling" function but not any menu entry to enable , The APP can check the hidden function and enable it if can use.
How to use the APP?
1.Start the app and click "Wifi Calling Setting"
2.Click "WLAN calling"
3.Click "ENABLE" WLAN calling
4.Click "WLAN preferred"
5."Wifi Calling" is enabled!
We welcome users to report compatible and incompatible phone models, and incompatible phenomena, we will further improve the APP.
Please send us an email: yangyz20191101@gmail.com

A full list of carriers that have added support for Wi-Fi calling -

Canada:Bell,Telus,eastlink,Rogers,Fido,Virgin Mobile
United States:Alaska GCI,T-Mobile USA,Appalachian Wireless,Unlocking,AT&T,Metro PCS,Ting,C Spire,Simple Mobile,Verizon Wireless,Sprint Wireless

Austria:A1
Belgium:Telenet
Czech Republic:T-Mobile,Vodafone
Denmark:3,TDC,Telenor
France:Orange,Bouygues
Germany:O2,Telekom,Vodafone
Greece:Cosmote
Ireland:Eir
Liechtenstein:Swisscom
Netherlands:Vodafone
Norway:Telia,Telenor
Poland:Orange,Play,T-Mobile
Romania:Orange
Spain:Orange
Sweden:3
Switzerland:Salt,Swisscom,Sunrise
Turkey:Turkcell,Vodafone
United Kingdom:3,O2,Vodafone,EE

Australia:Optus,Telstra
Hong Kong:3,SmarTone,1O1O and CSL,China Mobile Hong Kong
India:Reliance Jio
Malaysia:Digi,U Mobile
Singapore:Singtel
Taiwan:APT,FarEasTone
Thailand:AIS,True Move,dtac

Argentina:Claro
Brazil:Claro,Vivo
Panama:+Móvil
Puerto Rico:AT&T,T-Mobile USA,Sprint Wireless
U.S. Virgin Islands:AT&T,Sprint Wireless

South Africa:CellC,Vodacom
Israel:Partner
Saudi Arabia:Zain
United Arab Emirates:Etisalat

New support:Redmi Note 5 Pro

To check and enable the hidden Wifi Calling (VoWifi)  function