My account is being debited with an incorrect higher amount.

I have been on email and phone call with MTN since May this year. They are debiting my account and I do not have a contract any longer. My contract ended at the end of July and my monthly payment was R359, MTN is now debiting R549 from my account. Surely that is ******* because I did not agree to that. I call and call and call sometimes I get cut off sometimes I am on the line for one hour at a time. All I want is my MTN Billings department to contact me, to correct the mistake and credit my account which they have ********ly debited.For this reason I will not take out a cell phone contract again.