Thanks to Voice over IP, this is pretty much impossible to regulate any more. The telephone networks were built on a trust model that VoIP destroyed. Now, you can originate a call on the internet that's nearly impossible to trace.
Why do the phone networks route calls with faked originator numbers. I can see why they don't block genuine numbers (that terminate at an actual phone) -- to get you to pay for blocking. But surely they can tell that an incoming call from a foreign call centre with a local number is fake.
Recently I've had calls, in the UK, from numbers where the [given] originating number doesn't exist. Why any phone company would route these is beyond me. (They're faked because we're signed up to do-not-call lists and they don't want to be sued). More to the point is why the regulators (OFCOM) let the phone companies get away with routing such calls.
Now, when you call me, the HS gets "the call". It then contacts the VS. The VS will assign a temporary phone number to my cell, and send this number back to the HS. The HS then sends this number back to the switch making the call. That switch will then call the temporary number, hitting the VS. The VS sees that number and since it assigned that number to my cell, knows to "ring" my cell phone.
And that's just for cell phones. That doesn't cover call forwarding, for instance.
[1] Number portability adds another layer of indirection to this.
Unfortunately the phone #'s are always random and AT&T just wants to sell me a service that allows me to block up to 30 phone numbers. But they're calling from more than 30.