MTN -Incorrect Call Charges

I am on a MTN100 subscription whereby I get R 100 from the 11th of each new month. On my invoice of 11 January 2018, I was charged for calls even though I have adequate recurring airtime balance available. My airtime balance on 19 January 2018 was R 202.28. If I had used up my airtime from the previous month, my airtime balance on 19 January 2018 would be a maximum of R 100. I have not made any international calls, or calls to 086 and 087 numbers. I have called the call centre (ref nos. 1- ********** 9984, ********** 79) , but all they inform me is that my airtime was used up, therefore I was charged. I dispute this, as I have more than enough recurring airtime as at 1 February 2018. MTN should rectify their errors, and not give lame responses. I will be reconsidering my MTN subscription.