HDFC Bank — Cashback not received for an iphone purchased under the 5000INR cashback offer

I bought an iphone last month using the HDFC credit card and i was supposed to receive 5000INR cashback to my card. I havent received it and the customer care hung up on me after asking me to wait. The second time on asking the same to an executive who called me to ask if i had any queries about the credit card, he told me he would call me back. But no response was received.
I could not even find an email id where i could register a complaint on their website. Please help.