Lied to and billed for a phone that I was offered for free

I have been trying to resolve the purchase price offer for a phone and plan change from the Tmobile for months with NO resolution. What differs with me is I have copies of the "hand" written documentation, the worksheet and notes, from the store manager and TMobile will not acknowledge them. I went back to the store where this phone was purchased when the first bill came in and the manager, who performed this scam, gave me a $60 "Inconvenience Credit" and said she would correct the error. The next month when the bill arrived the same problem existed. When I returned back to the store but it was closed and I went to another local store and they informed me they would investigate and correct the problem. After multiple phone calls to this TMobile representative with no updates , only delays, I resorted to text messages to document this for updates. It appeared she did not appreciate this and i got more delays and the problem was not resolved. So after multiple phone calls to the Cares department that went on deaf ears and I should send a letter to TMobile Customer Relations @ PO Box37380 Albuquerque, NM  87176-7380 I sent they a letter describing the problem I am having and included copies of the handwritten documentation and text messages and after one month did not receive a response. I contacted Cares about this and they said they would investigate. I received a response from them and they determined they could not help me. My next question is: When is a company NOT responsible for the actions of their employees? Shouldn't they be!  This is not only what I was told, but I have the written documentation to back it.