Because the number is spoofed (or a VOIP), and the FTC has admitted they haven't been able to keep up with the tech. That info is posted on the FAQ page on the DNC site. Blocking is all you can do.
Don't know anyone in the 307 area code, so I did not answer and they left no voicemail. I can see that the National Do Not Call list is pretty much useless. I get this type of call on my cell phone at least once a week since I got the cell number 3 months ago.