Active Level 1
It's a technology to make a call over wifi. The technology has recently implemented by the two carrier provider Airtel and Jio. The Purpose is to avoid call drop, even to use at weak signals.

The Charges for the VOWIFI Calls are based on Unlimited plans only, If you have recharged other means it will be chargeable as per tariff (e.g: jio for IUC Charges, like airtel basic Talktime plan). 

In my point of view if they gave the VOWIFI data additional to the recharge, it will be fine. Airtel has initially provided this but now I'm not sure.

And the Main thing is Most of the Samung Devices are VOWIFI AND VOLTE Compatible.

If you have updated the OS to latest Android 10. You will get an VOWIFI Support at Dual Sim Cards also ( If the Carrier Supports).

However, Technology is good to improve signal strength and more.