Billed R14719.31 on data contract not in use

IN LATE APRIL 2015 I WAS CONTACTED BY VODACOM INFORMING ME I WAS DUE AN UPGRADE ON MY DATA CONTRACT ***) ON APPLICATION OF THE UPGRADE I WAS TOLD I HAD TWO CONTRACTS,THE SECOND ONE BEING ***) AND I HAD APPROX. 800GB OF UNUSED DATA ON THIS NUMBER ***) I REQUESTED THEY CANCEL THIS CONTRACT AS I WAS NOT USING IT AND WAS HAPPY TO FORFEIT THE UNUSED DATA.. I WAS SINCE BILLED R14719.31 ON THIS CONTRACT. AFTER MANY CALLS LATER AND VISITS TO VODACOM THEY HAVE SENT ME A STATEMENT SHOWING ME THAT I HAD IN FACT HAD SIM CARD *** IN MY ROUTER AND THIS DATA WAS USED. I DISAGREE AS I HAVE ONLY BEEN USING SIM CARD *** FOR WELL OVER A YEAR WHICH MY ACC. WILL REFLECT. VODACOM INSIST I AM LIABLE AND THERE IS NOTHING ELSE I CAN DO. THEY HAVE ALREADY DEDUCTED THIS AMOUNT FROM MY ACC. I CANT HELP FEELING THAT SOMEHOW I HAVE BEEN ****MED, ALL I WANT IS THIS TO BE CHECKED THOROUGHLY AND PROVE THAT I AM NOT LIABLE. MY EXISTING ACC.NO. WITH VODACOM IS I***-2, WHY AFTER SO LONG WOULD I SUDDENLY START USING A SIM CARD I WAS UNAWARE I EVEN HAD. PLEASE ASSIST.