Billed an ridiculous amount

Hi , my name is Patricia Oliver im a vodacom customer for some reason i received a sms`s from FNB that vodacom billed me R994.94 . I called and asked for explanations they gave me some answer of DATA usage of R423.86 and they cannot explain. i took a contract with vodacom and since day one am i encountering problems with them , my monthly payment is R199.00 , i blocked the line on R50.00 as its an open line, and when i reach that limit vodacom soft block my line , so i would call them if i need them to give me more airtime and i will be aware of it , but i buy Data if i run out of Data , but now they billed me R994.00 on a R200.00 contract and they cant explain it , and without recieving a warning sms or statements from them ,please help