Airtel — STD balance got deducted

I had recharged rs. 28 twice on 6th June, 2009 on my airtel mobile.
So there was an offer saying that if reach rs. 28 we will get rs. 38 STD talk time. I should get 76 minutes since I have got it done 1 rupee STD.
I hardly talked for less than 20 mins. When I was trying to call, it's saying that my account balance is too low to make calls. Pls. do the needful.
My Mobile Number: [protected].