T-Mobile USA review: Formal complaint regarding incorrect billing and unresolved customer service issues

Whom It May Concern,
I am writing to formally file a complaint regarding my final T-Mobile bill and the way my account was handled.
My problems began when I was charged for a modem that I had already returned. I contacted customer service multiple times over the course of approximately two weeks. Instead of receiving meaningful assistance, I was repeatedly transferred between representatives who focused more on asking me to complete five-star surveys than on resolving my issue.
After numerous calls, I finally reached a representative who acknowledged the issue and reduced the charge by approximately half. Wanting to move on, I paid the balance that was presented to me and ended my service with T-Mobile.
The following day, I switched to Verizon. At that time, the only remaining balance on my T-Mobile account was approximately $386, which represented the amount still owed on my device.
I have now been billed $903, including approximately $600 in equipment charges. This is confusing because I was told the phone was free through the promotion, and I do not understand why I am now being charged these additional equipment costs. It appears I am being billed for promotional credits that were applied while I was a customer, despite the fact that I fulfilled my obligations up to the point of leaving.
After everything I experienced with the incorrect modem charge and the poor customer service, receiving this unexpected bill has been extremely frustrating. I do not believe these charges are fair, and I respectfully request a complete review of my account.
Specifically, I am requesting:
A detailed explanation of every charge included in the $903 balance.
A review of the approximately $600 in equipment charges.
Confirmation that my returned modem was properly credited.
Any billing adjustments or corrections that are warranted based on my account history.
I have always made a good-faith effort to resolve these issues through customer service, but unfortunately my concerns were not properly addressed. I am now asking your corporate office to review my account and provide a fair resolution.
Thank you for your time and attention. I look forward to your prompt response.
Sincerely,
Linton Gallow III

Ps Throughout this entire process, I was made to feel as though I had done something wrong, as if I were trying to avoid paying what I legitimately owed. In reality, I was the one being charged for equipment I had already returned and for equipment I never used. I spent weeks trying to get these errors corrected, yet I was repeatedly billed for charges that should never have been on my account. It is frustrating and unfair to be treated like I was at fault when the billing issues originated with T-Mobile. I have acted in good faith throughout this process, and I expect T-Mobile to do the same by correcting these improper charges and providing a fair resolution.