The problem is rather very serious in big cities like Delhi. I never faced a indoor signal issue in my home town but Delhi was always a hit or miss. Few years back it was kinda okay as I could have an Airtel and a Vodafone SIM and it was very likely that one of the two had coverage. That problem became compounded as I/C is no longer "free" as we say.
Jio is better Indoor coverage, stable Volte calls, better battery backup, even for Iphone also. Airtel leads in speeds, drains battery backup due to switched between networks (4G to 2G & 2G to 4G) in Indoors.