Reliance Jio has introduced wifi calling now. Here is how you can enable and test the wifi calling on your phone:
I am using LineageOS 16 Custom ROM on the Xiaomi device.
Step 1: go to settings
Step 2: select Network & Internet
Step 4: Select Mobile Network
Step 5: Select Wi-Fi calling under "Calling" Section
How to test the Wi-Fi calling working on your phone:
Step 1: Enable Flight mode
Step 2: Switch on wi-Fi and connect to the wifi network.
Step 3: Try calling as normal.
If Wi-Fi calling is working, you should be able to call without an available mobile network.