They call and never leave a message. I am also on the DNC registry. Obviously they don't do anything to prevent this from happening.

The DNC has no ability to prevent calls from reaching you. It works when registered telemarketers remove numbers on the DNC list before making sales calls.  Violations are only handled after the fact.  But I bet you are not receiving sales calls from all your local insurance or real estate or travel agent offices to give an example so it is having a beneficial effect.

calls several times a day and hangs up

Tried to call them back on a blocked callerID phone and their machine promtly refused. A scammer or telemarketer for sure. And if not, certainly nobody I want to talk to.  22/sep/16.

Calls constantly but never leaves a vm. I called the number back and it's for a sweepstakes for a 50.00 card no thanks.