Thanks again, nickybrg.
Unfortunately, the calls come from UK numbers relating to areas where we DO have customers - London, York, Nottingham ... probably bought from some VoIP provider.
So although your suggestion is of course a good one, it could lose us business in our case.
I once got really shirty with a cold caller, quizzing him quite severely like a journalist for some time. Eventually, he said: 'Is there an adult I can speak to there, sir?'
I couldn't help laughing, complimented him on his ingenuity, and thanked him for his time.