VoWiFi has been rolled out to select
Samsung and
Apple handsets only. My IP has been working on Wi-Fi since the past few weeks...Sat 7th Dec '19 IIRC. And unlike Airtel's VoWiFi which works primarily with Airtel broadband, Jio's VoWIFI works on any broadband line.
The phone received a new firmware (13.3) and
carrier setting update (40.1) after which it was enabled. You will need to wait for Jio AND your phone's mfr to enable this feature. Plus, do remember it has been rolled out in some telecom circles only.