Ridiculous Interest

My account was handed over by Vodacom to VVM Attorneys. I had a problem with the phone, tried to sort it out but kept being sent back and forth between the iStore and Vodacom. The phone cost no more than R5000 but VVM charged me R9440. I did not dispute it because I didn't know any better, I made a payment of R5000 the first month, R500 p/m thereafter without missing payment. They call me twice at 7am a few days before payday every month reminding me to pay up and when I ask why, they advise that they want me to select the debit order option. I keep telling them no. I've tried blocking their number, but they keep using different ones.

I was left with R2000 to finish off the debt in November, I call to pay the remainder in December. To my surprise I get told I owe R4500. I ask why is this, I get told "its because I refuse to select the debit order option". This is making my life difficult. I moved to new city and I am struggling to find accommodation because VVM blacklisted me, I recently got into a car accident and need to replace it but I cant... VVM blacklisted me.

I am desperately looking for help or guidance on how I can proceed with this matter, are they well within their rights to go on like this?