Create an SMS A9 and sent to 121 .
After you sent the above message from your Airtel mobile number you will get the message says "We have received the request for A9 and it will be processed shortly".
You balance must be above Rs.9 when you sent the A9 message to get the free talktime of Rs.25.If you don't maintain balance of Rs.9,you will get the message says "Insufficient balance for REFILPROFILE. Minimum balance required for refilprofile is 9.0 and your current balance is Rs.."
The free recharge trick has been tested myself in Tamil Nadu on day before yesterday and worked for me.The one and only drawback of the trick is you can use this talktime to call only Airtel Mobile Number .The validity of the free talk value is till tomorrow midnight when you activated.To check the balance of Airtel to Airtel free calls then dial *121*7#.Remember this trick doesn't work if you have balance below 9 rupees.