It will only come back to voice mail if you set up 'remote coverage'.
Disp cov remote:
Input outside number in one of the free spaces, remember the space number.
Create a cover path; 'add cover path next'
For the first point of coverage, put r+the number of the space you inserted your outside line in remote coverage, so if you put 9p14035556767p678 in space 10, you should type "r10" in the first point of coverage space.
The second point of coverage should be your voice mail hunt group.
Adjust the rings to 2 or 3, anything more is way too much.
Let me know if this works for you.