“WiFi calling” this phrase sounds new but it's not.Messenger,whatsapp,Skype,imo and many others use this technology, baked in their respective apps and call it as calling feature, with costs internet data rather than currency.
Now the same technology has pushed its limits,broad enough,to fall out from apps to your devices.It's now baked in device and carrier to replace,at least to some extent , traditional calling technology .

The advantage here is; when ever you go through the sessions of weak or no signals you can rely on WiFi hotspot to make calls ,and this come with better sound quality than traditional calling.
The difference in WhatsApp ,Messenger,Skype,imo is; you can make call as an identity of these platforms ,unlike the indentity as your  cellphone number.

Already there are many illustrations of its adaptation in USA,but this lacks in INDIA.The requirements to adopt this is ,both your device and  carrier should supports WiFi calling .
Many devices across the globe support this feature but the carriers at their particular locations lack in adopting it in them.
In INDIA the emerging carrier JIO is envisaging to deploy this feature for supportive device, among which GOOGLE PEXEL stands at first position to get this