Deception of promotions

What the supervisor Jonathan suggested is, you cannot just believe what a representative tells you. How did this all start? I called the tmobile customer service and asked if they have a new promotion that I can add a line and get a line free. The agent said "yes". Over the time of the same conversation, I asked the representative three times, "So, I am paying $30 for a line and getting a second line for free, right?" and he said "yes".
After I consented, a couple of days later, I go and check my bill, and I am being charged the $30 twice without any notice. So I call again, and they tell me that the representative who made the mistake is going to call me back the next day, but he never did. After a week I call again and have to explain it all over again. I ask to talk to the supervisor "Jonathan", and I tell him to go and listen to the tape about what I was told in the original conversation. He says they made a mistake and they are not going to make it right, and they are going forward to charge me double.
In conclusion, never trust what the tmobile representative tells. Always double check!
What Jonathan also did lie about is that there is no way of filing a complaint online. He said that the  company does not have any online complaint sources, and gives me this wired P.O. box address. I just needed to google once to find many means of online contact. These are too many lies to be an accident.