Mtn is billing me for a phone i have long finished paying for

I had a contract with MTN (R479) which ended in September 2017, I have not renewed or cancelled my contract, which means I am on month to month contract and that means I should be paying less then half of the initial contract which was R479, but in the month of October MTN charged me R619 of which when I contacted them they promised to fix and I have just been informed by Mtn that this months bill will be R489, kindly assist me