T-mobile charging me after cancellation

I was not informed about the prorated bill or anything. When I asked the customer agent, the agent said there will be no charges and I don’t need to worry about it. I don’t understand how a corporate T-mobile handles business like this, telling lies and not informing their customers of accurate information. I did not get any notification about the additional charge and suddenly I get a bill from the Collection Agency. This is a major issue. Please remove the record from the CCA asap. I am going to take this matter to BBB and legal lawsuit if it does not get resolved.