Write an email to their customer service stating your phone number and your address and ask them to enable VoLTE. That is the same way I did it. But VI VoLTE is not stable and not available in all parts of India, so I would suggest you to switch to Jio. I did the same and I am getting excellent VoLTE services.
first take phone into any 4g volte supported phone and call to activate volte. they raise your request. now take your sim in m30s and after 24 hours volte is on in m30s. I have also try and now vi volte run on my phone.