Paid CA in 2004, Original Creditor says they were never paid

In 2004 I had a cell account that went to collections. I received the bill from a company called ERS. The total bills was $1073.XX. This was WAY too high so I called the cell company. They said my balance was only $500 and within a week I received an amended statement from ERS. I paid ERS 2 - $150 payments, 1 - $100 payment and 1 - $100+ payment to pay off the account. A few weeks after the last payment, I received a bill for $450 which is what they had originally taken off. I did not pay that balance as I knew I had paid off what I owed.

Fast forward to today. I am going thru a divorce and wanted off my husbands account. I tried to set up new service with the same cell company and they came back that I still owed $450+. I asked the rep how that was possible and she said they only ever received a payment of $50. They never received anything else from the collection agency.

It has been 5 years. I don't know what bank account I paid this debt off with as I have changed banks multiple times since then. The cell company told me they will never let me open an account with them until I pay it. I can't determine which ERS they used and neither can they as they are no longer affiliated with ERS.

Do I have ANY options here?