I also think this is an issue that started with Android 8 update and is also present in Android 9, my current Android version. It has been going on for a few weeks. My voice mail used to operate just fine.
Thanks for the suggestion, and I did try entering the APN manually. It was already listed correctly except I couldn't enable it (grayed out). When I entered it manually, I had the same problem of the grayed out Enable/Disable.