Mtn being ******

My wife had an mtn cellphone contract. Each month she noticed she phoned allot and her bill was always over R1000. Then she went into the store and asked to put a R200 limmit on her calls. Then it worked for a month and the month thereafter her bill came up to 1400 she then paid it and went back into the store told them to put on the R200 limit which they did. Worked for a month or 2 and then stopped working and got a bill of R1600 which she paid again. She went back into store and then put the limit on again. When it failled the 3rd time she refused to pay the amount and ended the contract. Now we have been struggling for long to get these dept collectors to stop calling. Explaining to them over and over the issue and them saying that they will look into it. Which never happens. Im now getting tired and sick of this