It's a bit of a kludge, but I can accomplish this on my phone (Android) by enabling "Do not disturb", "Allow exceptions", "Phone calls + Messages", and setting

It's a bit of a kludge, but I can accomplish this on my phone (Android) by enabling "Do not disturb", "Allow exceptions", "Phone calls + Messages", and setting "Calls and messages from" to "Contacts only". I'm under the impression that it's also possible to replace the dialer/call app with one that can detect "probable spam", and at least one such replacement can likely do custom ringtones or disable ringing for such spam.