Free phone promo not honored

I added a new phone line upon the free phone promotion on iPhone 8 64GB in early October 2019 via AT&T app. After 3 months when credit starts to kick in, the third bill only shows $10 credit instead of an amount equaling to the monthly installment of $15 on the phone. I called customer service and they said the day I added the line, there was no such promotion but a different one. I was arguing that was not correct because the free phone promo was the reason why I wanted to add a new line. Claiming that after verifying the promos in the past, they did not honor that promo. I regret that I deleted the confirmation email (who wants to keep it a few months after the activation???). I think there was something wrong in their system but the customer service agents never want to accept that. I've been their customer for more than 13 years but this is the first time I feel like I was lied and cheated by AT&T.