When is voicemail used?
If voicemail is available and enabled for a hunt group, it is used in the following scenarios.

Unanswered Calls
When a call has rung unanswered at all the available hunt group members. If overflow is being used that will include being unanswered by the available overflow group members.

Queue Limit Reached
If queuing is being used, it override voicemail unless the number of queued callers exceeds the set Queue Limit. By default there is no set limit.

Night Service
When the hunt group is in night service with no Night Service Fallback Group set.

Out of Service
When the hunt group is out of service with no Out of Service Fallback Group set.

Who Receives Message Waiting Indication?
By default no user is configured to receive message waiting indication when a hunt group voicemail mailbox contains new messages. Message waiting indication is configured by adding a Hgroupname entry to a user's SourceNumbers tab (User | Source Numbers).

Accessing Hunt Group Messages
By default no mechanism is provided for access to specific hunt group mailboxes.

Phone Manager
User's with hunt group message waiting indication can access the hunt group mailbox through Phone Manager.  

Intuity Emulation Mailbox Mode
For IP Office systems using Intuity emulation mode mailboxes, the hunt group extension number and voicemail code can be used during normal mailbox access

IP Office Mailbox Mode
For IP Office mode mailbox access, short codes are required to access the mailbox directly.

Broadcast
The voicemail system can be instructed to automatically forward messages to the individual mailboxes of the hunt group members. The messages are not stored in the hunt group mailbox.