How do I get ATT to correct my bill

I have been trying for 3 months for ATT to correct my BOGO discount.  I have spent over 10 hrs on the phone trying to get help, constantly get transferred to another department that asks for same info and does not help.  I finally got a case #, but they have not done anything with it. Case established June 1, reviewed June 3 and is still open.  I am so frustrated that they cant fix a simple issue with my bill.  I am considering switching providers since the customer service is aweful.