Repeating billing issue

I had a billing issue at the beginning. I called and the agents fixed it and promised it won't come back again. Unfortunately, it came back every month as other customers observed and reported here. 

This month, it became worse as the agent always said they will escalate to their manager even it is nothing new. The wrong bill has not been fixed yet. I guess none from T-Mobile will take it serious as we just call them back again and again. Even it is small amount money, but it is annoying it becomes every month issue. Any suggestion for a legal procedure we can pursue? Is anyone willing to join me? I would let T-Mobile to learn a lesson if they treat customers like this. I know this is a T-mobile managed website, some popular contributors maybe hired by T-mobile to "fix" issue here. They just recommend us to call again and again to waste our time and hope we will give up.