AT&T is double charging me for my iPhone and accusing me of lying to them when I try to fix it
In September 2016 I purchased my iPhone 7 from Apple. I have had the phone line for 10+ years and have the 2016 receipt for purchase of this iPhone. AT&T is claiming they have no record of this. They are instead claiming that I purchased my iPhone and activated my line for the first time at a BestBuy in February 2018. This is factually inaccurate. I have never bought or activated a phone with BestBuy.
I just got off the phone with AT&T and they basically called me a liar, saying they the phone's IMEI clearly shows that I bought the phone from Bestbuy in 2018 even though I've had this phone since 2016 and the number long before that. Like I have toooons of records showing that I've had the number for over a decade, yet AT&T is claiming that I activated it for the first time with BestBuy in 2018. It seems so easy to prove, yet they aren't interested. They've been charging me the full price for a new 2016 iPhone 7 starting February 2018 even though they would have sold for significantly less in 2018. They could make a simple phone call to Apple to verify the purchase.
I honestly do not understand how AT&T's record keeping can be so bad that they can't see that I've had this line for over a decade. They basically told me I'm out of luck and out the $700+ in extra device fees that they've been charging to our family plan. AT&T has effectively robbed me, and are acting like I'm the one trying to scam them. Their records are plain wrong, and they won't look at my Apple receipt which clearly shows that I purchased the phone in 2016.
What recourse do I have?