ATT lost my trade in phone and are charging me for it

I purchased a new phone and mailed in my old phone for trade-in using the pre-labeled box provided by ATT.  I mailed it over a month ago, following all instructions.   Now, ATT states they never received the phone and they are going to charge me $350.  Clearly the process that ATT is using to get and process trade ins does not work, and they are charging customers for their problem.  I spent over an hour on the phone with them this morning and they continue to say that since they can’t find the phone I won’t receive the trade in credit.  We will be switching to a provider that has decent customer support and integrity.   Customers should not be paying for the ineptitude of ATT.