In addition to this website for obtaining information on unknown callers, I also use http://www.spydialer.com. Between the two, I am almost always sure whether I need to block a number or not.
called my DNC listed #, left no VM when I did not answer. Only Scammers and criminals do that. Real people and businesses want a call back so they leave a name / #. Criminals / scammers do not leave information because then law enforcement could find them easier.