Get a line cancelled and refunded for service I never had.

In July 2022, I purchased a phone from Best Buy and then returned the phone 2 weeks later to Best Buy.  I am currently being charged installments on the phone I returned and I am getting charged for a line I do not have.  I have contacted Best Buy and ATT about the situation.  I have sent in my copies of the receipts and return receipts and the issue is still there.  Who do I need to contact to escalate this issue as I keep getting no where with basic customer service?