Bad communication and bad service structure

So I am a new mtn client,just took out a contract about a 2-3 months ago. First month I paid my bill like I should and also the P/R rating as well. This month I got my bill like I should and they decided to deducted extra money of my account,and when I went to the store to ask why,I was told it is because I had some arrears from the previous months, but I paid everything,and after that they deducted the exact same amount again after I asked about it,and I have sent them an email to help me understand why they deducted almost R1500.00 for a R800.00 contract to months in a row.The agent I spoke to told me to take her email address and send her the proof, because according to the system,I was in arrears,and yet if you look at my bank statement I paid everything and more and it has been more than 3days + and no response as yet. I am not one to complain,but some of us work our asses of just to earn some money to get us by and yet some people,just don't mind that.I would like to know how is it that We as customer's have to wait so long for a response from MTN but if we decide to switch to MTN they are quick to help and assist.