Disputing bill, has now resulted in blacklisting

My mom signed a My 2 Gig contract with Vodacom and Altech Autopage for 149 month including a wireless modem on the 06/07/2012. Every month she paid the same amount as it is a standard my gig 2 gigs. But then in her December 2012 statement she then gets told her bill is R14 278..... we tried everything emailing the bosses of the branches, My mother went into the store where she signed the contract. Everything and everyone just said well sorry you have GPRS charges that amount to R12385 excluding VAT. Which we find absolutely insane to be charged that amount when it is a standard contract, its not a cell phone contract and there are no extra charges. Then all of a sudden its an insane amount of R14 278 including VAT. She paid her contract amount that was due every month until the contract expired of R149 a month and having no one help with this insane bill they have now handed my mother over at the age of 60 she is no blacklisted for a bill that I just find absolutely insane and totally ridiculous. She has never been charged GPRS charges throughout her entire contract except that once and never used the 2 gig again after December. So I'm at a loss, I have tried everything....