Every 3-4 months.airtel will pit some charges in your post paid bill.
Kindly check your bill regularly else you will be paying more.and they will keep you charging till the time you high light them.
They will activate some or the other Value Added Service.and then will charge you hevaily for the same. When you will contact customer care, the agent will not listen mentioning it has been activated by internet automatically even if you have not requested.
Then you will keep on fighting with them for the reversal.
Also, their services are going down.poor call connectivity, lot of call dropping.Go for BSNL and MTNL, even they are better now.poor internet speed. They will communicate 7.2 mbps in 3G, but you will get 7.2 kbps.
in 2G they will communicate 256 kbps.but your will get 2.5 kbps speed.