It is up to Jio to support VoLTE on Windows phones. The phone is capable.
T-Mobile in the Czech Republic supports VoLTE on Windows phones with their app and SIM with no update from Microsoft required.
Link to their VoLTE information. I can not provide a translated page because it is secure https. https://www.t-mobile.cz/volte
They support the Microsoft Lumia550, 640 LTE, 650, 950, 950 XL directly, no firmware update required, just their app.
The problem is with Jio