Complaint

Hi, I'm writing with respect to my account with Vodacom, I took a phone at Vodacom, I pay on the last day of the month, I called them to confirm if they debited and they confirmed, after few days I called again to confirm same they also confirmed, I then received a call saying my account is outstanding, I paid 2 x the monthly installment plus a late payment penalty last week, they just called me again this morning saying there is still two months which hasn't been paid for, and I also have to pay a penalty for that payment even though I enquired if they debited, they're refusing to take a responsibility and now I should be the one paying.