Harassment by Telkom

Telkom refunded me +-R1000 of which I have proof, but to my surprise they are now through the company called Nudebt harassing me saying that I owe them, how do you refund a client and from there you turn around and say the same client is owing you. That's harassment at its best, I went so many times to Telkom offices and all what they say is that I don't have have to worry but on the other hand they send their little dogs to called Nudebt to harass me.