How to file complaint about billing issue

Well, you are not alone. Five years ago I had T-Mobile I went into an office. I changed my plan the next month they build me correctly but the month after that, they started billing me back at the higher amount. I went in and canceled my T-Mobile account because I do not trust them. I purchased a Solex device that is only supported by T-Mobile so almost 3 years ago. I had to reinstate my T-Mobile account. In March 2025 I closed one line on my account the Solex line. My bill was 130 with both lines and home Internet. The following month I got billed correctly $80. In April they bill me for two lines $130. This is the exact same thing that happened four years ago. When I called them, they said no no you just had a credit. I said no I closed the line and you can see that I didn’t use that line and they beat around the bush. Finally offered me a credit to satisfy me which it does not because I’m closing my account. When I got off the phone with them and looked up complaints oh my gosh, there’s thousands of people who has had the same thing happened to them. This company has zero integrity. They’re practicing fraud by billing incorrectly and acting as if it’s an error on our part when it is intentional on their part. It didn’t happen once it happened twice and then it happened. 1000s of people. I am closing my account with T-Mobile. I choose not to do business with greedy dishonest business practices that bites the hand that feeds them. I suggest you all do the same so that we can put this dishonest company out of business.