First I was using vodafone worst service in metro and at my college there is no signal then I port my sim to airtel it takes time but I can use 4g in metro or even in college basement if your phone is 4g then switch to airtel 4g by swapping sim for free.
You can use 4g data plans on 3g(the main advantage) but **its call rate are too costly** and other features that airtel have is airtel money you can pay to various operator by only one no.(you hgave to only load money)
This review is 100% genuine