DEBT FNB

I received for the past two weeks many calls from a collection agency claiming an amount of R20,000 from me for a debt that I ,according to their instructions, supposedly owe to FNB. I told them that I am not a client of FNB and they must sent me proof of the debt to enable to investigate the matter. On the 15th August 2022 someone called me again and mentioned that he is from FNB and claiming payment of the debt. I asked him to send me proof and he answered that he can't send me any proof as I must get it from another department of FNB.