Plus, in Australia at least, the calling party pays a premium to call the mobile phone (essentially charged at a long distance rate, to a separate 'mobile phone

Plus, in Australia at least, the calling party pays a premium to call the mobile phone (essentially charged at a long distance rate, to a separate 'mobile phone' area code). So in reality, spam callers self regulate anyway: they know what area codes are mobiles and they know that those area codes will cost them.