Mtn
They are debiting my acc unlawfully

I got an sms on the 9/01 from mtn that said I owe them R6674. I I called immediately and Mbali told me she doesn't know what its for but will solve it. Ever since then I call everyday with no valid explanation. Mean while I am getting bouncing debit orders from them trying to debit an amount I know nothing about. I spoke to Sanlot Nkiwane who is the manager on the 11/01 who promised to sort the matter and never called me even once. His excuse was he asked someone else to do it. Now they sending me smses that said my line will be blocked if I don't pay. The last person I spoke to (Moshiladi) said I bought a handset from them but can't confirm at which outlet and failed to send me the invoice or proof of the purchase they claim I made. I also emailed their customers care and its been more than 4 days now still I have not gotten a reply. Not even a simple call to let me know they are working on it. Please assist