I have never had good experience with using VoWiFi, explicitly have it turned off on everybody's phone at home because it was like your average VoIP calls - latency delay while talking and the other person hearing.While both do support Vowifi, for some reason Airtel seems to be more twitchy. But I think it has more to do with my device (Poco X3), as my sim on another device has vowifi working flawlessly, Dad's Airtel also has vowifi without issue, both devices being Android One. Since you will be using on iphone, don't think vowifi should be an issue.
I think other members experience with vowifi should help here.
Is it more phone or network operator dependent?