Corporate store refusing refund

My mother has been trying to return a phone at a corporate at&t store within the 14 day grace period. She was never given the packaging, just the phone and charger (no box, earbuds, or paperwork that typically comes inside the box). They are refusing the refund on the grounds that she does not have the packaging. I have called customer care multiple times and everyone I have spoken to has said there is no reason the store should be denying the return. However the store just simply will not do it stating that customer care is wrong. What other options do we have to escalate this issue? It is not fair for my mother to be treated this way, being called a liar, when it was the store's mistake to begin with.