Called number back from a trac phone to see what it is. It is a scam about winning a cruise to the Bahamas. Just another scam trying to get people's personal info.
Here's a way to take your method a step farther. Set up a GROUP or FOLDER named XSpam, and add all of your "Xspam" contacts to that group/folder. Then assign a unique ringtone to that XSpam group.
That way, every member of Xspam group is called, your phone will ring with that special ringtone, allowing you exhibit all the indifference and/or animosity the caller so richly deserves. (OK, so I'm biased). You can keep on doing whatever you're doing without worrying that you're missing an important call. (A self-satisfied giggle is optional)