Billed after contact expired

I paid up my contract of R159 in Dec 2016 but they are now billing me R39 of which I don't know what ito is for. I blocked their debit order and now I'm afraid they might blacklist my name for nothing. The sad thing is I used their sim card for only 5 months. I regret doing business with them now and for this experience I will not advice any of my family or friends to do business with Telekom ever again.