Hi friends here is a great Tricks for Airtel 3G User.
First activate Airtel 3G ( if you you use 2G SIM) For Activation send SMS 3G to 121 You will receive a SMS stating, 3G service is already active on your number. to choose your plan replay with 1.1. Data Plan2. Deactivate 3G Choose 1 for DATA PLAN
then Choose 13 (it may change for different circle) for 4 GB free AIRTEL 3G DATA. You will get 4096 MB of DATA and Rs. 1.00 Main balance for free. ******** Try this with low balance.