Ok, here's my understanding and hopefully someone else will chime in if I'm off.
When a HG member is logged in and doesn't answer their phone, the call will forward to the "leave" action of the hunt group voicemail box. When no one is logged in, it's considered "busy" because there's no queuing. If I'm thinking along the right track, you would want to enable the queueing, set the queue time limit to like 2-3 seconds. Then in VM Pro (assuming you're using VM Pro) you'd create a "Queued" module under the HG name and basically point it over to the "leave" module.