How can you do this via settings in Android? I've spent hours searching for the answer and have yet to find it.

Add spammers phone as a contact, then select "all calls to voicemail" for that contact.

That's not a very useful solution if I have to do it once for each spammer. You wouldn't accept that for e-mail.

Spammers use a spoofed caller ID with the same first 3 digits as your phone number so it looks like a local number sO blocking them by number doesn’t really work.

I agree with you. I've tried to block each one of them for maybe months, but nothing's changed. I still get at least 2-3 calls each day from them. I think ignoring them is the best option for us now, or if you would, just answer their calls, ask their company name, and sue them because of their multiple calls. I just read about this at http://www.whycall.me/news/my-4500-payday-from-a-telemarkete....

So let's make a daemon app which calls that exact number on the 2nd SIM and if it's not busy drops both calls.

You can't add "Private Number" as a contact, nor can you block it.

Even pre-smartphone landline systems could block (or route direct to VM) numbers with blocked caller ID.

You absolutely can block such numbers in the stock Android phone app; some online sources indicate iOS lacks this basic feature, but I don't know if they are accurate and current.

How do you block calls from numbers with blocked caller ID in the stock Android phone app? I can't find that option.

“block unknown callers” in the call blocking settings.

That doesn't exist in the latest version of the app. If I go to Settings->Call Blocking it's just a button labeled "ADD A NUMBER" plus a list of blocked numbers. The ADD A NUMBER button triggers a popup with an input field that only allows phone numbers.

Spammers generate random numbers in your area code for each call though, they don't call from the same number twice.

Can't. You can block individual numbers, but there is no tool in the base Android to block all unknowns. Fortunately there are plenty of apps which will do exactly that - but I'm confused as to why I need to install an app to do something so simple.

Because if any core OS had to contain every feature which is deemed "simple" then that OS will become bloated and complex.

The solution someone suggested elsewhere in this thread is to use make the default ringtone silence, then assign audible ringtones to all your contacts.

Well “blocking” isn’t the same as “not ringing”, right?