The airtel has started a service of whatsapp recharge in a simple way. You can recharge whatsapp data by sending just sms from your mobile and 43 rupees will be deduced from your mobile. The amount may vary as per your region/area. Prior sending the sms you must have 50 Rupee balance at least. You can enjoy whatsapp with minimal charges, need not to recharge any internet pack. After a successful recharge only thing is that you must have minimal 1 rupee balance in your account, so it would not stop working.
If have SIM of Airtel, Type Whatsapp from your mobile and send it to 121
As you send you will get reply to confirm it, Type WA and send it further to 121. Your whatsapp recharge will be activated, it will work till you have data limit.