Unkown Contract

I went to telkom shop rustenburg to buy a new prepaid sim n data and got a reciept.saturday i got an sms saying i owe telkom R2788.00.I CALLED them today and they told Me i have a new contract on the prepaid sim i bought.Now they dont want to cancel the contract immidiatly they want to launch an investigatigation n want me to pay that amount.how did they open an account without my documents.please help