GETTING BILLED MORE BECAUSE OF A USAGE AMOUNT I DID NOT SET

I AM SO ANGRY AT THE INCONSIDERATE NATURE OF MTN AND THEIR BILLING SYSTEM I HAVE A CONTRACT WITH THEM THAT COMMENCED ON THE 1/04/2016 AND I HAVE PAID AN AMOUNT OF R319 A MONTH AS PER CONTRACT DETAILS AND I HAD SET A R0.00 LIMIT AMOUNT AS I PREFERRED TO BUY MY OWN AIRTIME AND DATA AFTER THAT THEY PROVIDE ME WITH WAS DEPLETED. EVERY SINGLE MONTH SINCE APRIL LAST YEAR THIS HAS BEEN WORKING WHEN SUDDENLY ON 1 NOVEMBER TO MY SURPRISE ON MY BANK STATEMENT I HAVE A CHARGE FOR A MISSED A DEBIT ORDER AND WHEN I CHECK I REALIZE IT IS MY MTN ACCOUNT EVEN THOUGH I HAD ENOUGH FOR THE AMOUNT THAT HAS BEEN DEBITING SINCE APRIL BEING R319, I OPEN UP MY STATEMENT AND I SEE THERE IS A BILL OF R348 DUE TO ADDITIONAL USAGE I HAD ON MY ACCOUNT, I HAVE NEVER EVER SIGNED UP, PHONED OR REGISTERED FOR AN ADDITIONAL USAGE, WHEN SPEAKING TO THE MTN CALL CENTRE AGENT HE TELLS ME THAT I HAVE AN ADDITIONAL USAGE LIMIT OF A R100 WHICH IS UNTRUE AS PER MY CONTRACT AND AS PER MY OCT STATEMENT IT SAYS CLEARLY THAT MY DATA USAGE LIMIT IT R0.00 I WILL NOT PAY THE AMOUNT ADDED TO MY ACCOUNT AND I WANT MTN TO PLEASE REMOVE IT, IF THEY CANT I WOULD LIKE THEM TO PROVIDE ME WITH EVIDENCE AND PROOF OF WHEN I EDITED THE DATA USAGE LIMIT TO R100. I HAVE MY OWN CONTRACT AND STATEMENT READY AS MY PROOF OF THE CONTRACT I WAS CONTRACTED TO AND THAT IS WHAT I WILL PAY. I WILL NOT PAY ANY ADDITIONAL AMOUNT, MTN MUST FIX THIS ASAP BECAUSE I WILL NOT GET A BAD PAYING RECORD DUE TO YOUR LACK OF COMMUNICATION TO ME AS YOUR CLIENT AND EVEN WORSE FOR ADDING USAGE AMOUNTS TO MY CONTRACT WITHOUT MY CONSENT. MTN FIX THIS PLEASE I HAVE A LONG RELATIONSHIP WITH YOU AND THIS IS MY SECOND CONTRACT I WOULD HATE FOR ME TO CHANGE MY VIEW OF YOU AS A PROVIDER DUE TO PURE I COMPETENCES AND LACK OF COMMUNICATION