I'm having issue with customer support false charges with no result

a customer service representative tired to lie to me on the phone and remove a line and add a line and i caught it because they sent  me a service agreement that i didn't sign. This has costed me a charge on my bill of non return fee of 401$ customer support said they would give me a credit of 370 witch doesn't make sense since the charge on the statement is clear, customer service will not let me speak to a manager and they have lied about removing the charges and made excuses to why is i have to pay this amount. I'm a business customer for 10 years and was a T-Mobile employee for 4 years. Please any advice before i cancel with T-Mobile and move on to another carrier.