MBD collections and attorneys harrassing people who don't owe money to companies

These attorneys and collection agency has been harrassing me and my father about a cell c account that Is not in arrears. They call from different numbers and we have blocked them as far as we could. Cell c confirmed there is no arrears. But these people are relentless and they slam down the phone or threaten us with legal action because we refuse to speak to them any more. We have mailed statements to them confirming all payments have been made. These people ...this company needs to be taken out of business.