Standard bank harassing me for paid up debt

I had overdraft with standard bank many years ago. I changed to fnb and my account was handed over to agencies for collection. I paid off every cent and issued with paid up letter by the agency of their choice. Now Standard bank is harassing me with call they want payment. I tried explaining but still I'm still getting harassed for the account that was paid many years ago. Why can't they go to the agency where they handed my account over to get info or some sort of payment clarification? Can I please ask that Standard bank stop harassing me for debt that I don't owe.