Incorrect billing

I received a call on 24/2 from above advising me that my cell account is in arrears with R12 834 since August 2014. I do not get any statements as they post them. I pay an amount of +/- R1000 into my account every month as that is what I use. Apparanty it is for data that was used on one of the other phones on my account that has not been used for about 2 years now. apparenty the amount that was billed to my account in September of R12 000 was incorrectly pd by another customer, that is why I was only informed on 24/2, when the other customer that pd the R12 000 investigated his account. What i cannot understand is that the other customer only after 6 months discovered that the R12 000 did not go off his account in September, and Autopage did also not pick it up or block his phone for overdue money. Please this is rediculous. I do not even have the simcard of that number *** anymore.