T-mobile charging me after cancellation

Hello, I previously had cell phone service with T-mobile and we cancelled it and switched to ATT in September 2021 because we never had good service anywhere. My last bill was autopaid in September and never received any letter from T-Mobile. However, I received a letter from a debt collector named Convergent dated 1/10/22 for T-Mobile for $623.91. Then, I received a Final Notice letter from T-Mobile dated 1/20/22 saying that I owe $623.91. I called the debt collector and they said T-Mobile pulled the account from them. Then I received a letter from a different debt collector named Frontline Asset Strategies for the amount of $1,710.41. I disputed it with Frontline. T-Mobile has been charging me monthly for service that I cancelled in September 2021. How can they charge me for service that I don't even have? How can they try to send it to a debt collector, pull it back from them and send a different amount to a different debt collector before trying to contact me? I will proceed with a lawsuit if this account is not cleared to a $0 balance and cleared, as I have read online many multiple complaints of T-Mobile doing this to other customers. Gracias.