What kind of voicemail do you have? What kind of huntgroup is it? How many people in the huntgroup?
This is why I said it was vague.
One example how this could work:
You have an incoming group with receptionist in it and voicemail turned on, receptionist logs out of group, should send call to voicemail because no active users.
If you have 50 users that is not a reasonable solution. That is why the more information you provide the better answer you will get.
The truth is just an excuse for lack of imagination.