Voice over wifi is currently supported only by Jio on the S9 Plus. If you are using Jio make sure you have the latest Android updates installed. I guess once the device is upgraded to One UI 2.0 wifi calling should work on Airtel as well.
VoWifi calling is working fine on Jio and airtel depending upon the location u r putting up. Also the network provider doesn't opened the access in all areas in india.
