Has called my cell phone several times just to hang up when I answer.

Has called my cell phone multiple times, often 2+ times a day.  Never leaves a message if I don't answer, and when I do, no one's there, and the call terminates.

Calls several times a day - no message - no answer on pick-up.

I answered the phone and said hello, and about 3 seconds later someone hung up

I tracked it down online. It's a collection agency. I'll bet they are telling their clients how many times they call to try to collect and charging them per call, but just have some automated system making the calls without contacting people. Sounds fishy. We can go to their website and email them to stop calling. http://www.naaco.com/