Depends on some conditions to use proper wifi calling: 1. enable voLTE in your phone. 2. enable wifi and wifi calling. 3. only some operators give these facilities in some cities. depends on where you live. only some devices supports it.
I was also facing same problem. but now problem has been solved . you can try with jio sim it will work. becoz airtel is currently supporting limited devices to support this features, but In coming days airtel sim will also supports.