Generally an answered call where there is no one on the other end is a tracker call, that is, a computer that dials a number to see if gets an answer. Just picking up the phone and saying anything such as "Hello" or "Yeah" or some such is sufficient for the computer to register that your number is a good one for follow on calls, human or ROBO. December AARP magazine mentions this tactic is being used quite a lot.
It would seem a mystery who or what is calling except for the fact that they keep using 1) same prefix, 405 241 0xxx, 2) the same CNAM of "Okla City, OK," and 3) that each number is VoIP serviced by Peerless Network. Here is a quick rundown of calls using CNAM and prefix:
405241 0766 week of 2OCT,
405241 0790 week of 25OCT,
405241 0791 week of 31OCT,
405241 0688 week of 18NOV,
405241 0764 week of 29NOV,
405241 0998 this week in JAN2016.
A blog from one of those previous numbers identified the caller as representing the International Association of Police Chiefs. Believe it or not, this is a real organization. I'll leave it to someone else to verify whether they are 501(c)3 qualified, registered in state of Oklahoma or have an IRS tax exempt number.
But it is very likely that the International Association of Police Chiefs does not actually make these calls. It is probably a promotions company hired by this association, which is why you see location instead of their real CNAM or even the association's caller ID name. Some cancer organizations (such as Breast Cancer Society) have made that mistake of hiring a promotions company not realizing how aggressive these companies can be.
If you really wanted to know all that background, then there you are. But when you distill all that info, it all comes down to this: either get a blocking device or give up your phone number.