It's All About the Money
As a coupla folks already mentioned, the phone companies really want spammers to continue. Just like the mail service loves "bulk mail" The Phone Company loves spam callers.
I know this is true because I heard about the company also mentioned above, that has set up a successful business to combat spam calling. I attempted to sign up for their service, but there was a problem: the whole scheme revolves around a special feature I needed to add to my phone service. My landline phone provider does not and WILL NOT ever implement this feature. Even though they would be billing me for this feature every month. I do not recall their name for this feature, but this is what it does - you add a phone number that you want to ring every time your land line rings - this would allow you to forward all your landline calls to your cell phone, for example. In the case of the spam fighters, you add their number instead. Then, they receive the same Caller ID information as yoiu do. When your phone rings, they will intervene and answer all calls on their spam list, and politely decline on your behalf. One (ring) and done!
The real beauty of the system, though, is that subscribers become a sort of honey net. When spammers begin calling from a new number, the pattern of calling, as observed over the subscriber base, quickly reveals that that number is now a spam source, and it automatically gets added to the spam list. For the curious, the service also provides each subscriber with a white list, so that schools and other legitimate bulk callers can continue to reach you. I think it's brilliant.
But there is no way for me to subscribe, since land line service is one of many monopolies I have no choice but to deal with or go without. And, yes, I am very close to dropping my land line forever. And in fact, this is exactly where they want me to go, because land line service is both more expensive to provide, and less profitable than cell phone service. They really, really want to convert me to their cell service. Not that I don't already have a cell phone, so my actions are almost entirely contrary to my best financial interests. Take that, artificial intelligence!!