Unhappy Client

I took out a contract with Telkom around March or April. As per standard procedure , they ask for all my details ( ID Certified, recent payslip and proof of residential). All of that was submitted at the Mall Of Africa branch . My contract was approved, even so after a whole hassle . The sales consultant was very good and patient with me and explained the monthly debits and how they had a penalty fee if you miss a payment. My contract was approved and I paid on time according to my knowledge. I even called the debt department to arrange for them to debit my amount as of the 20th of every month way up until the last day of the month and we had an agreement. So why is it that just after missing one payment (when there was an agreement to debit as of the 20th) now Telkom is threatening to block my Sim and even so how does my account be in arreas for almost R4000. Please make me understand because when funds were available in my account, Telkom did not deduct. And now all my debit orders were all sorted besides this one yet I am basically being blacklisted for something I don't even know. Please help clarify this because I am so unhappy that anytime from now I won't have a working phone.