T-Mobile cancellation of cell phone. Beware of penalty even after contract is over. Internet

Ok Here is the deal.  I wanted a nice rate for data, talk, and text that I have friends getting from T-mobile for $50 bucks a month.  I have been a loyal customer for many years and have completed two seperate contracts with this company.  They could not give me a deal under $100 dollars a month.  So I decided to look elsewhere.  I found Straight Talk and the deal is $45 dollars a month.  But you have to load a card on the phone.  Still not a bad deal.  So I changed today.  However, t-mobile never said that I would still be billed for a entire month billing cycle.  So I called to speak to the represenative who could not explain why this was happening.  I asked to speak to a supervisor and when asked the name of the person, she hung up.  I don't think they realize that by doing this she has lost a customer permanently.  I will not return to T-mobile if I don't like Straight Talk.  There is Sprint, Att, and many more to choose from.  So far Straight Talk which uses Verizon Wireless towers is a good switch.  We will just see.  I think that T-mobile should improve their customer service, so that they don't make their customers irate.