Supposed Debt Harrassment

I had a cell contract that ended July 2017. Last year my monthly rate of R400 jumped to around R1200 each month, obviously I could have gone over by buying data ect. but every month by that much? I was even in the Netherlands for two months, my roaming wasn't even activated how is that possible?

Regardless I paid it cancelled my contract at the end of July and received a confirmation message from Cell C. Two months after that suddenly Cell C tries to deduct R1290 from my account, I stopped payment. Followed the dispute procedure as they specify on there website, the only feedback was a message that asked if it was resolved, I rep**** no and didn't hear anything further.

Suddenly I start receiving threatening messages about payment deadlines to avoid legal action. What the hell is going on at Cell C. If they were smart they would have just taken the exorbitant amount of money they, I suspect, overcharged me for the year of 2017 and left it there. I refuse to pay another R1290. Cell C has pushed their luck too far...