Airtel volte has been a pain. I have jio volte that works fine. As far as i know, airtel volte only works on nos and oos.
Please check in the respective threads.
You can also post this query in the official beta los 16.0 thread, as everything here is forked from there.
It works in MSM-Xtended 8.1 & 9.0 too... Have not tried Havoc.