extra billing

Hi.ive recived a contract cell c phone as a gift frm my inlaws.the contract usualy comes to an amount off R380 to a max of R520.the statement for august is an amount off R513 nt to my surprize the amount deducted was R972.i call the call center and dey have no idea y i was deducted that amount.i am veri upset as i have to cough up the extra R459 which i didnt plan for.billing shud have a statement on the right figures.and debts shud go accoding to that.i need help to sort this out