Unnnesary Payments

I am not happy with MTN.. how they conduct the business.. I took out Month to Month contract.. My bank didnt allow the debit order the 1st time to go off.. So so i making payment at the store monthly but now after 3 months they saying that i must pay R2000 because the 1st debit order never went off. I have been up to to date with payments from the start.. Please sort this out..