I also received a call today regarding yes bank limit enhancement. It was an automated IVRS call from a landline no. of mumbai.
It asked me to press 1 if i wanted a limit increase. I pressed 1 but next it asked me for the OTP received . (It was an OTP from yes bank only which read"Otp XXXXXX is there to proceed for your request" , so i didnt punch that in, it seemed fishy. I am not able to find the offer anywhere else. Strange.