bad attitue, harrasment and invasion of privacy

the last four weeks I have been receiving calls from DMC on a debt fromstandard bank of which I had a loan with but had been paid, apparently the debt had been sold to DMC, I am unsure how they have gotten my contact details but I am getting harassed at home and at work. I have advised the rude call centre agent that they are not allowed to contact me on basis that there is no debt, as I have checked my credit report aswell, she has persisted for me to give a reason and blatantly said that the numbers are generated and I will definitely get called until I give reason as to why, which I have advised on. this could be detrimental to my credit record, and I will take steps accordingly. according to the National credit act, <br> it is ******* for a company to sell a prescribed debt, or to collect a debt which has prescribed, \where the consumer raises the defence of prescription