Wrong Information with regard to my billing

A WHILE BACK I FELL BEHIND WITH MY MONTHLY SUBSCRIPTION WITH MTN. THEN I CALLED TO ARRANGE A THREE MONTHS PAYMENT PLAN FOR WHAT I OWED ABOVE MY NORMAL SUBSCRIPTION. THE ARRANGEMENT WAS MADE AND I WAS TOLD TO PAY R1100 FOR THREE MONTHS AND MY ACCOUNT WILL BE UP TO DATE. I THEN AGREED WITH THE TERMS AND PAYED AS AGREED. ON THE THIRD MONTH WHICH WAS DECEMBER I CALLED MTN TO FIND OUT HOW MUCH I OWED SO I CAN SETTLE THE AMOUNT, I WAS THEN TOLD BY THE CALL CENTER AGENT NDIBUWHO THAT MY ACCOUNT WAS ACTUALLY AHEAD AS I HAVE BEEN PAYING MORE THAN WHAT I SHOULD SHE TOLD ME TO ONLY PAY R780 F0R THE DECEMBER MONTH OF WHICH I DID AND SEEING THAT MY ACCOUNT WAS UP TO DATE I THEN INSTRUCTED HER TO PUT ME ON A DEBIT ORDER AND SHE ASSURED ME THAT I WILL PAY MY NORMAL SUBSCRIPTION THE NEXT MONTH. COME NEXT MONTH I WAS DEBITED OVER R1500 BY MTN AND WHEN I INQUIRED I WAS TOLD THAT IS WHAT I OWE. NOW, THIS TO ME SEEMS A BIT SUSPECT. I WAS TOLD MY ACCOUNT WAS UP TO DATE AND NOW ALL OF A SUDDEN I AM MADE TO PAY OVER R1500 IN JANUARY NOGAL. THEY THEN TOLD ME THAT THEIR AGENT DIDN'T GIVE THE CORRECT INFORMATION. WHO'S FAULT IS IT THAT THE AGENT IS ***********? THE INCONVENIENCE CAUSED NEEDS RETRIBUTION!