I feel T-mobile is charging more then what I should be charged.

I enjoined T-mobile in November 2016, during Black Friday.  I traded in a iphone 6 128 GB for a iphone 7 plus.  I told I would have pay for a certain to get the Iphone 7 plus and the cell was mine.  I wasn't told I was going to be charged, then give a credit of 20.00. When, i realized, I was being charged for the iphone and give the 20.00 credit.  I called T-mobile to inquire charges and resolve the issues with my cell phone bill.  It has been several times I tried to resolved the issues with my cell phone.  The first time I was told I was not paying for my iphone, although, I was being charged and given  a credit.  After, when I tried resolving the issue with my cell bill.   There time I got different responses that wasn't the same answer, I got previously.  I rate has been changed by T-mobile,  I was paying less then 110.00 and now I am paying 130.00.  My rate has changed four times.  There are few times, i have add other accessories and cell phone.  When, I tried discussing the issues with them, I felt they didn't want to acknowledge they changed the rate of the plan to a lower rate then higher rate, then back to a higher rate.  When I also when into my account to history.  It doesn't  go back to the beginning when I joined T-mobile to dispute what rate from the beginning, I had started with in my plan.  Today, I was told I needed to go to the store to find out what  was the rate of the plan, I had started with.  I asked why can't they check their records, I was told, the information wasn't in the system.  In other word, I have to go to the store to get the information myself.   The representative had told me, she would pass on the information to management and  I should or may hear from them.   What should I do