MTN billing more than agreed

May normal debit order of R 733 with MTN is always scheduled to go off on the 1 of every month. As i have 2 cellular numbers with MTN and have been a loyal client to MTN for the past 8 years. My contracts as always is set as a capped data usages what that means that if i do run out of data that i need to purchase more data bundles howver on the 1 January 2015 without consent or any bill send to me MTN deducted an amount of R 5500 of my account which in return caused all my other debit orders to bounce dueto lack of funds in my account..... After numerous phonecalls and visiting the MTN branch in pavilion they adviced that i can arrange to pay this amount off on a monthly basis and that they will investigate only without any proper service or help.. This is without me every increasing any limit and for the past years i have been paying the same set amount every month.....