No refund for phone after changing carrier

Not sure if anyone out there can help, but here goes.

I recently changed carriers, leaving T-Mobile.  When I called to close my account, T-Mobile customer service rep was awesome, walked me through shutting down my account, stopping autopay, and help me set up a "quick and easy" return of my Apple iPhone.  He generated a return label and packing slip for me.  I followed the directions to the last detail and returned my phone.

My phone was received in the T-Mobile warehouse on 7/25/23.  T-Mobile confirmed that yesterday via phone when I called them to verify they'd received the phone.  Why did I call to verify that?  Because I got a bill from T-Mobile for $675.00!!!  T-Mobile is saying I am outside the "Buyer's Remorse" window and they owe me NOTHING!  

Okay, this is not a case of buyer's remorse.  I was a T-Mobile customer for probably 15 years!  I simply wanted to change carriers.  When I closed my account, the customer service rep never mentioned anything about buyer's remorse, never indicated I would not receive any credit for the equipment I was returning.  

I was a customer in good standing - paid my bills on time via autopay, upgraded my equipment regularly, never filed any weird claims.  And now they want to stick me with a $675 charge for a phone that is now in their possession.  I don't get it.  

So, here's my question:  what now?  What recourse do I have?  I've tried speaking again to customer service twice on this matter.  Finally today, I received an email stating, "sorry, your loss, too bad, nothing we can do".  I am stunned.

Any suggestions or guidance are welcomed.  Thanks a bunch.