INCORRECT BILLING TO MY ACCOUNT AND NON-RESOLUTION OF REFUND ON CHARGES

on the 20th of November i purchased 1Gig data bundle from MTN for what i thought was the normal amount of R189.00 that would be billed to my monthly account. Upon checking the balance end November as i was preparing to make the EFT payment the voice prompt confirms i woe R5,300.00 , not my normal balance of say for arguments sake R1,500.00. I immediately call the call centre, they admitted their fault . I even received an sms saying the error has been cleared up. Upon following up with them last week Monday and again today, the credit is still pending!! i will not pay these incorrect charges, no one can give me a clear answer, i must just sit back and wait. it is unacceptable. to have to wait fot three weeks to correct something that is not my fault is ridiculous and the service is pathetic.