I was charged after I switched my service and now my bill has been sent to collection.

I was a T-Mobile customer for 12 years. I swtiched to Verizon in June of 2023. I called T-Moble to port over my numbers and disconnect the service. I kep getting automatically charged for the service that I was not using. A T-Mobile customer service rep helped me clear the charges until October of 2023, she assured me that the account was closed. I returned all the equipment that I had from previous years. I got the bill for november and it was sent to collection recently. I called TMoble and spoke with a rep. called Mitch. She did not resolve my issue. Can you please guide me onhow to resolve the charges that I incurred for service that I did not use? Gracias