I have tested it with Nokia 5130 and it works for me. You can use this trick to get legally 2GB/Month for 2 months means you will get 4 GB usage for 2 months.
For this you just have to send an sms from your mobile
Send ‘DATA’ to 543210
This plan will be activated within 24 hours of sending message.
It will work on NOKIA phones only, so if you have any other handset other than nokia than just put your simcard in nokia handset and then send sms DATA to 543210, after receiving reply from airtel put your simcard back to your phone and you will be able to use it in your phone too.
If you found this trick not works for your handset try to send this message with another handset.