How to file complain against Tmobile

I replaced a line to my plan and was told that I can trade my old Iphone to a new one without any charge. First, Tmobile increase my month charge (even though I was told repeat the month charge should be same, yet they increase without my consent. A nice rep finally fixed the problem. Tmobile also charge me addition fee for the new phone (again I was told it is a even trade in). Now they refuse to resolve the issue. I have been with tmobile for 20 plus years and now I am really angary being lied to. I plan to file a complain with consume protection agency. Anyone else experience similar problem?