T-mobile unable to trace returned phones

I returned unopened phones to T-Mobile. The UPS tracking shows that the items were returned to and accepted by T-Mobile. However, T-Mobile is still charging me monthly for the phones, even though I have returned them.

I was told to wait a few months for the issue to resolve. I have made numerous calls, receiving different answers each time:

1. First call (T-Mobile CSR): I was told to wait a few months for it to resolve. They credited me for the first month.
2. Second call (T-Mobile CSR): I was again told to wait a few months for it to resolve. They collected the UPS tracking information and tried to check their warehouse.
3. Third call (T-Mobile CSR): The CSR suggested that I might not have sent the items.
4. Fourth call (T-Mobile CSR): I was advised to call UPS and file a complaint. UPS confirmed that the items were delivered.

T-Mobile acknowledged that the package was received but stated they are unable to track the IMEI numbers and that I have to pay for the items. I need to know what exactly they received and acknowledged.

If this issue does not resolve, what are my options? I am planning to file a complaint with the BBB.