I don't answer calls from unknown numbers, under the assumption that if it is a legitimate caller they'll leave a message.

This entity called me three times within a nine minute span, yet they left not a single message...

If the subject of these calls was legitimate , and important enough to make so many calls in such a short span of time, one might think to leave a message.

This latest contestant in the ongoing game of scumbag phone-tag doesn't meet the criteria for a returned, or even an answered call.  However, they can take solace in knowing their number did qualify for addition to my call blocker.