Unsatisfactory service -MTN

I am disappointed with Mtn, as a customer we expect efficient service and no inconvenience however last month MTN deducted R1900 instead of R1200, I personally went to the Store at Menlyn Mall,Pretoria and the consultant reassured me that he logged a ticket to rectify the mistake and went on to assure me that only R500 would be deducted for the following month. The problem is persisting as this month I was billed wrongly again, amounting to R1900, this is such a disservice to me and uncalled for, this is against contractual agreements and obligations

MTN should fix this or I will take it further.