How to get T-Mobil to honor their agreements

The same thing is Happening to me.  I was offered off 200.00 on top of the promotion and to pay a certain amount per month.  I saw it didn't reflect what I was told on my bill so I called to be told they could not honor a representative mistake all they could do was retrain them.  I have been with tmobile since it was voice stream in 1998. It's sad any customer would have to go through this.  I was at the tmobile store doing an upgrade was there for 4 hours. I had to call tmobile support and was offered a bigger deal for all I had went through. I called today to be told they see the notes and could not honor it Because it has to be part of the promotion. Spoke with manager who only wanted to do an eip for 2 months making it 60.00 off. I told him the day I called i was offered 70.00 but asked for a manager that what a coincidence the phone call dropped.  I called back with another representative that assured me she would take care of it and offered me the bigger discount so I accepted and went forward doing the upgrade.  I have now requested to speak with this manager supervisor to call me tomorrow see if he is able to assist me further. The last Manager stated that's all I can do we can't honor what a representative informs you. I also informed manager when I was offered the xtra amount off I had on speaker and the instore tmobile that was assisting me at the time also heard what was offered. Manager stated again even though the instore heard the conversation they still couldn't honor it. I'm like how is that.  Someone needs to make this right.  And this has happened to me on 2 other occasions.