incorrect amount debited with no explanation

i ve got a contract with mtn they debited 1791.00 on the 20/06/2015 of which i dont know why because they normal debit 463.00 and i have asked the to set my limit to 0.itried to contact them there was no answer .visited empangeni branch they didnt see anything until i reversed the money and pay the normal installment 463 but what happened my account went to arrears and thy debited 1820.00 in july .that when i visited them again and they asked me to call 808 of which they told me it my other number 071 that idont normally used it .it was 25th of july when i asked them to email statement .until today i havent received it . and i cant even get hold of them.they must please respond this is not acceptable really