I see this is an older thread, but it is close to home for me.
I have nothing to do with the product other than being a very satisfied end user, but there is a service that catches incoming spam calls upstream and sends them to a group of bots, artificial voices powered by IBM Watson. The bots pretend to be human, keeping spammers talking in circles for as long as possible. When the call ends, the system emails you a MP3 of the conversation. It is often absolutely hilarious. The bots pretend to be interested, but after a while they start saying things like "Shhh, there is a bee on my arm" or "...so I was just making homemade bathtub cleaner out of ammonia and bleach, have you ever tried that?" etcetc.
It uses TrueCNAM to detect spam calls, and lets you select a weighted score, so you can set your own level of sensitivity. You can also manually whitelist and blacklist, both by direct number, and by wildcard. You can build a whitelist by exporting your contacts to a .CSV then importing it into the system.
Personally I have a wildcard filter set up, because the weight system favors specific numbers. With a "blacklist *" set up, only your whitelist can ring your phone.
It works with LL, SIP/VOIP, cell and Google Voice. It uses either conditional call forwarding or simultaneous ring.
Best IMHO is to port your number to GV for $10, and either use an Obi device if it is a home phone, or use GV on your cell (configured to "use my normal phone app to make/receive calls with GV"). Just never give your new cell number to anyone, and all your outgoing CLID will reflect the GV number. It is transparent.
With a cellphone, the issue of a single ring leaking through is easy to fix, just use a custom ring tone that begins with 7-8 seconds of silence. With the Obi device, you can set a string to delay the ring, but you do so at the cost of seeing the incoming caller ID. But either way, if using a wildcard blacklist, you can be sure that if your phone rings, it is a call you want.
I set this up for my wife's parents who live in an assisted living home, because they received a ton of spam calls from the typical con artists. Now only family members and close friends can call them. It makes us feel safer.
Again, I don't mean to pitch a service - I just love it, and scammers are a big pet peeve of mine. I don't want to even leave a link, but if you google jolly roger spam or something like that, you will find them. It cost $7/year when I first signed up, but I think it is about $12/year now. Bonus points - find their YouTube channel and listen to some of the calls. Even if you have a spam solution you are happy with, you will laugh your a$$ off.