This is a first, so I'm grasping at straws. Do you have a test system that you can set up to see if it acts the same way. Maybe if the test KSU works fine maybe you have a software problem. Seems odd that the phone still stays in the group, but is skipped. Like I said maybe you can try to recreate the problem on a test KSU. Just a quick question are the hunt groups attached to a CCR option in the voice mail?

Here is another tip from the book to look at.

Hunt group set numbers cannot be in a ringing group

Hunt group set numbers cannot be in a page zone.

It does not explain why those two things are important, but its worth a shot.