Airtel — prepaid recharge of 28 days gets over by 25th day

I have been a customer of airtel for 12 years now and i feel cheated by the company now.

I recharged my prepaid connection with the 199/- offer o[censored]nlimited local /std calls for 28days, on the 20th of april and today, the 14th of may which is only 25 days from the date of recharge i get a message from airtel saying i have low balance and need to recharge. I feel cheated. I want this resolved immediately.