Oh, and a calling line ID thingy helps too. If it says 'overseas number' I don't answer, if it says 'private number' its Oz based and shouldn't be calling, making them fair game for violating the do not call register.
Of course, there are times when I'm feeling downright nasty and go on the attack straight away. I would have thought people working in these call centres would have thicker skins than some of them have ;-)

