Probably politically incorrect and certainly not a valid sample but in my experience, most fraudulent callers at least have a strong Indian accent and try some sort of Microsoft support scam:

http://www.wired.co.uk/news/archive/2013-04/11/malwarebytes

The time difference would explain the prevalence of night owls and other factors often used in fraud detection. Only on my direct office phone number, I receive 5-10 such calls a week.