Final bill payment/account pay off

Hello,
This is Ayad ***, I had an account with AT&T that I switched over to T-Mobile a few months ago. Since then, I called AT&T numerous times to send me the last bill so that T-Mobile can pay off the balance but I never received such a bill. Today, I received a letter from a collection agency for the total remaining balance. This is very upsetting as I mentioned, I was constantly asking for the final bill to be sent to me so that the whole balance will be paid off. Kindly, solve this issue as soon as it's possible. I'm writing another address for you to send the bill to down below:

***
Jacksonville Beach, FL 32250

Thanks.

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