Airtel Volte works fine for me. Am from Kerala. In case of Jio Volte icon appears instanly but for Airtel it takes some time. May be a minute or so. Try toggling Volte switch from Quick settings panel. Also try to reset network settings along with APN reset.Airtel, Kolkata.
The volte option is now present but even if selected, the signal does not show it.
As a matter of fact my phone doesn't even show 4g.