Vi Vowifi working on vpn.

Messages
320
Location
India
ISP
Airtel | Jio | V! | BSNL
Wifi calling does need the cell towers initially to determine what's your roaming status, before you can use it. I have tried with different carriers (Indian and foreign) to keep the iPhone in airplane mode, and try to connect only to wifi in order to make a call. I've not been successful. I'm on the latest iOS. Region is a country that supports unrestricted wifi calling. It needs an initial connection to the tower, and then once wifi calling shows up in the status bar, I can turn on airplane mode and switch on wifi only. It'll work.
 
Messages
320
Location
India
ISP
Airtel | Jio | V! | BSNL
@Gaurav_add Like I said before, it will once the location has been determined to be within India. If the phone doesn't have a location fix, it doesn't activate Vowifi.

Location in Smartphones is quite complex. Operators use a variety of ways including but not limited to, MNC codes/eNB serial numbers of the surrounding towers, GPS coordinates from handset, surrounding WiFi hotspots SSID's and their known locations from archived databases, etc. This is all in addition to the IP address of the access network, which can't be spoofed with an on-device VPN but maybe a network-wide VPN (such as one installed on a router) could mask it.

My point is once the location has been determined by the operator, it's safe to turn on Airplane mode and connect only wifi. Things will work for a while till the operator needs your updated location again. Which all sources (and priorities) your device and operator use to determine your location, the algorithm behind it, and how long that location fix is valid for before needing revertification is all operator dependent.