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We really can't. Technology has advanced so much that they aren't even actually calling from that number. The caller ID shows a representation of a group of numbers that the company has purchased. So you block one and another shows up later. You have to resort to leaving an outgoing VM saying something like:
"I'm sorry I can't take your call at this time. Leave your name & number along with a brief msg after the time and I will return your call promptly. Please understand if you do not leave your name, number and the reason for your call I will not consider your call a priority therefore you will not receive a call back. Thank you for understanding." Friends, family and customers (if you take business calls on that line) will understand because it MUST happen to them, too. The next weapon of defense is to recognize that when a call comes in with an unrecognized number, yet that call is VALID, remember to add that number to your contacts so you are aware next time. Even some VALID businesses will have multiple numbers and the best thing you could do is note them & enter them into your contacts.
Some phones will allow you to block the numbers completely and other systems will not put the call through but they will still appear on your recent calls list. It's a total pain in the ass.
When are the normal, everyday people going to be protected with technology and not be charged up the wahoo for the protection.
One of these days someone is going to have an accident running to answer one of these calls and they're going to be relentless when tracking them down and sueing the life out of them.