plus the problem is that not every company seems to list volte on their phones because it was not an issue until now. all new launches, the companies are highlighting support for volte. nexus 6p for example does not support volte right now because google has not released the required patch yet.
even gsmarena or mi website does not list volte... so the only safe option is to check with existing owners or the company itself on social networks. first option is of course more reliable.
assuming jio is updating the supported device list on their website regularly, that might be another place to keep an eye on.