If you are wondering how you can get Voice Mail to answer when your lines are busy, YOU CAN'T.
For that issue, you need your last line in your hunt group to have EXTERNAL Voice Mail.
Otherwise, when all of your lines are busy, your callers will hear a busy signal.
That's just the way it works.
Thus the need (At Times) for external voice mail.
I always make sure that my Doctor & Dentist clients have External Voice Mail, as well as Internal Voice Mail.
It's not so important for non-emergency related businesses.
Such as mine......