iphone Device Charge that is not mine

I purchased a new iPhone online which was lost by FedEx before it reached me (investigated and confirmed lost by FedEx and Xfinity).   I called Xfinity for them to send a new one, which they did after it was proven that FedEx indeed lost the first phone.  After receiving and setting up the 2nd iPhone on my account, I began paying monthly installments for it.  In addition, Xfinity started charging me monthly installments for the 1st iPhone that FedEx never delivered.  Now, 3 months later, I am visiting Xfinity retail and calling Xfinity customer service to get the lost iPhone removed from my account.  Each time I call or visit the store they credit me for the monthly charge and indicate it has been fixed.  But the next month the monthly charge for the lost iPhone reappears.  I have visited the store 4 times and made at least 15 calls to the overseas call center.  This is probably the worst experience I have ever encountered with a purchase.  On top of all of this, I was just informed that I have reached the maximum number of credits allowed.  Someone at Xfinity has to be able to fix this. Please help.