Incorrect Billing

On December 2014 I received my Invoice and it was incorrect MTN charged me for an upgrade I never do. and I sent an email to MTN Customer Care querying the invoice, after few days I got an email back asking for my contact number for assistance, I sent my number but no one ever called or emailed me after that. I then call customer care and the two consultant I spoke to didn't know how to help me. they refer me to upgrade department and to refer back to customer care again with no help. and few day later I called them again as I wanted pay my account and the lady I spoke to she was able to assist me, then I pay my account after then I started getting sms from MTN saying I have an outstanding amount on my account, I called MTN I was told to ignore the message as the request for the credit hasn't gone through yet. but when I receive my invoice in February I discover that they added the fee they claimed it was overdue, I called MTN again and the lady told me she will send a request for cancellation. but today I got a sms saying I must pay the money to avoid service suspension. They must call me not sending me sms. I want things resolved TODAY!!