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.