Trade-in scam, again!

Att lied again.

I did trade in my iPhone about two years ago for promotion credit. They have changed their words for two years, and now they are saying that my old phone has been recycled and they can't give me any credit.

It's my second time... I am sick of Att.

I have the trade-in reference number, confirmation email, 3 chat records, and att trade-in website also show that they received my phone, but it now shows processed.

With all pieces of evidence, they keep saying that they can't give me any credit!

What shall I do?