My wife used to work for a financial concern and they bought trunk lines in major cities to appear like local calls. Not hard to do at all. And perfectly legal.
Not-so-honest concerns can "spoof" the system with fake numbers. That is NOT legal from what I have been told.
Most personal phones can block only a certain quantity of phone numbers. And most of these clowns use MANY rotating numbers so it is darn near impossible to block all of them.
Best blocking system is your own common sense! And have fun with them while keeping them occupied on the phone call!!!!!!