Caller: I don't know, it's hard to hear them

this number will not stop phoning our office number, usually rings after hours and leaves messages on our answering machine but they have now started to phone during the day. They ring to say that someone that use to be here owes money. It is very difficult to understand them due to their accent, man and a woman and phoned. Wish as a business there was a way we could block nuisance calls on the landline.