Over paid for a line that did not exist

I no longer have service with this company. But, when I got a final request for payment, I happen to go through my bill and found that I was being charged for a phone line that had never existed. So, I did my research and went back to the first time this problem was created and sure enough, I was paying for this phone line for months.  When this first happened and I noticed, we had called and they took off the charge for the phone itself but not the service charges for this extra line that should have never existed. So, fast forward to today, I paid the last bill and went inside a store and they told me to call customer service. I called customer service and explained this whole complicated story again and finally she understood BUT, because it was a large amount of money they owed me, she did not have the authority to give me my money back. So, she connected me to a different department. But, that lady couldn't help because I am not an active account holder, so she transfers me to billing. But, somewhere in between there, we were disconnected after a long, long wait. 

I am just so frustrated that it's taking me this long to take care of it. Please Help! I really don't want to but, if I must then I will try calling billing again and hopefully someone wrote notes about my situation.