VOX blaklisting error

I cancelled my services, adhering to the notice periods - handing back the router by courier etc. -- now they go and list me for more than R6K - I am getting calls from a debt collector - when I ask for what it is they are not allowed to tell me .... I mean that is ridiculous as you want me to pay money and don't even tell me what I did wrong. I have sent emails and to no avail no one assists you.