Bad customer service

On 23 June i sent a complaint to MTN regarding them billing me incorrectly as i was on a TOP UP package. they responded and said Hello ThatoN126,Thank you for your post.<br> Your account is up to date with payments and it is on Top Up.We would like to assure you that we are actively working to improve our service levels.Regards Thabelo.<br> On the the 17th July i went to MTN store to advise them that my contract comes to an end on the 22nd July&they should migrate me to a prepaid package,the lady advised me that i owe an amount of R2750.97.When she went into my profile it showed that a Journal had been passed, and i didnt owe them.Then someone reversed it. i was advised that i don't owe because when i was charged i was on TOP UP&couldnt have run up a bill for data and calls amounting to R2750.97 when i was purchasing my own data and airtime. Can this kindly be rectified as now they are making their billing mistake my problem.I am now receiving threating sms's telling me that my line will be suspended in 2 days if i don't pay.Why am i expected to pay money i don't owe?I'm considering moving to another service provider that knows how to treat their customers fairly&with dignity.