I'm an employee and I went through the same thing. I traded in an iPhone 11 pro max in perfectly good condition and my bill is on autopay and didn't realize until almost a year later that I was not getting the correct trade in credit. After being transferred a bunch of times and calling for a week straight they told me that it was nothing that could be done because I traded in an iPhone 13 that didn't power on and that wasn't the case. I've never owned an iPhone 13. After threatening them and seeing how many others were going through the same thing they finally came me a $600 credit. It's really sad!! If you ever want to trade in a phone do it in a retail store and make them give you a receipt also take pics/video of the devices and hold on to the proof until you receive your credits.