formfollowsfunction
Curious Onlooker
In my case it does show on the phone that it has fallen back to 3G (the icon changes from LTE to 3G) as soon as I start a voice call when on LTE but internet actually doesn't work.Airtel is using CSFB (circuit switched fall back) for voice calls.
When the mobile recieves a call, it switches to 3G/2G. LTE being a pure packet switched network cannot deal with calls except when using VoLTE and therefore fallback to 2G/3G is needed in absence of VoLTE.
Nice document by Ericsson detailing CSFB: Source
If it falls back to 3G, data can be used. If it falls back to 2G, data cannot be used.