"Withheld" or "Unavailable" numbers tend not to be answered, (sadly some friends have their numbers withheld).
If we do find a scammer on the line (often using a 0203 or 0204 number to look believable) "Mr Levis" is in the garden and the caller is asked to hold while he is fetched. This often takes so long that the caller has hung up by the time that he "arrives". But their bill has been increased, and someone else has not been pestered.
There is a an oriental lady with a melodious voice who offered to reclaim bank charges from Barclays. When told that I was not a customer, she suggested, querulously "Natwest?" Not sure that she liked being told that she was a scammer., trying phishing.
Sometime I must try my limited German on one of these callers. Serbo Croat might be understood by some of the callers. Swahili, or Navaho could be a good mode of communication? MUST go on a language course!
Howard