Debt order after contract ended

I had a 2 years cellphone contract with mtn which ended in june 2015. My last debt order was supposed to be in july 2015 as it was agreed with the consultanted when i was terminating the contract.. The debt order went through in july 2015. But MTN is debting my contract for what i dont know, becuase my phone is on prepared now. I went to the branch, they consulted couldn't explain what the debt oder is for. And i haven't received a statement to say there's outstanding amount in my contract or something like that..can someone assisst becuase im getting charged everytime i reverse a debt order at the bank.