Steps to enable VOWIFI in one plus 3T : (Try at your own risk)

  1. Dial *#800#

  2. Go to oneplus logkit

  3. Go to function switch

  4. Check on Vowifi switch

  5. Reboot your phone

  6. Go to settings -> wifi & internet -> sim & network

  7. Turn on WIFI Calling

  8. Reboot your phone

It worked with airtel network.

Thanks to this post: link