I have my phones all on the Do-Not-Call list, but there are always those who ignore the law and spam anyway. They tend to do it from the same numbers, and I have added them to my phone’s contact list and associated them with a custom ringtone that is 15 seconds of silence so I never hear the spammers call me. Over the years people have asked me for this ringtone, which is easy to make in GarageBand, but here it is in iTunes/iPhone format:
Copy this file into the Tones folder in iTunes on a Mac or PC, then sync it to your iPhone by plugging the iPhone in, selecting the iPhone device in the left hand side-bar, then clicking on the Tones tab in the main window and sync all or sync the ones you want.
It is not perfect, as when the phone is on vibrate mode, I am still aware of the spam callers, but it helps my sanity just a little. A better solution would be for the phone carrier to let me BLOCK all calls from an annoying number; but they refuse to do that so far.