iPhone XS Trade In was Sent Back

I bought the new iPhone 12 Pro and traded in my pristine iPhone XS. The trade in phone arrived at the right location but was sent back to me without explanation. The really odd thing is I was given the credit for the trade in. Now I have this extra iPhone and don't know what to do with it. Do I send it back to them again? How do I get a shipping label? Do I just keep the iPhone XS? Thanks for any insight!