For some reason my question isn't getting the kind of responses I had hoped. Let me start with a clear statement of the issue:
1) Do Not Disturb was working fine on my iPhone 6 Plus (one of 5 devices in my home) until a few iOS updates ago, and was successfully blocking unwanted calls of any nature.
2) I have set Do Not Disturb to be on from 12:01 AM to 11:59 PM, essentially all the time. Only calls from those in my Contacts are allowed.
3) Repeat calls is off.
4) Sometimes the phone shows the moon icon, at others it does not, even though the settings remain unchanged.
5) A soft reset does not correct the problem.
6) A factory reset does not correct the problem.
My understanding of DND, and as verified by Apple support, is that these settings should keep unwanted calls, save those from certain agencies and types of businesses allowed to legally punch through. This is not the case.
Therefore, the following two assumptions are reasonable:
A) Do Not Disturb is no longer working on some subset of the iPhone population—likely some older devices.
OR
B) Telemarketers have found a way to pierce through DND.
This leaves only two conclusions:
—Item A, above, seems the most likely of the two.
—Item B, while less likely, is possible.
In either case it means Apple's iOS 11.x through 11.4 has a hole that needs patching.
I hope this makes my quest for a fix to this problem sufficiently clear:
If anyone has a solution, other than chucking out more $$ for a third-party app, that makes DND work as it should, and according to how Apple indicates DND should work, I'd like to know what it is.
As a footnote, I used both Nomorobo and Robokiller prior to locking down my iPhone with DND (when it worked) and found Nomorobo the better of the two as it effectively blocked most robocalls and works about 99% of the time in background where I do not have to do anything with it. It is also a buck less than Robokiller a month.