The docs say: An outside call that hunts never goes to voicemail and will hunt until answered or abandoned.
The rest is here and you may find a workable solution:
Group Call Distribution
A line can be configured to present its incoming calls to one of the 6 hunt groups. The incoming calls hunts from one hunt group extension to the next using the same hunting algorithm as used for an intercom call to that hunt group extension number. The call rings with the outside call ringing pattern and the display shows caller ID information if any.
If the hunt group extension that is chosen to ring as part of the selection algorithm has a line appearance for the line, then the call alerts on the line appearance with the standard slow flashing green LED indicative of a ringing call for me. Line ringing options are overridden and the line always rings immediately. Any other extensions in the hunt group with the line appearance, that have not been selected as part of the algorithm, will show the slow flashing red LED indicative of a ringing call but not for me. In addition, any other extensions in the system with the line appearance but not part of the hunt group, will show the slow flashing red LED indication.
If the hunt group extension that is chosen to ring as part of the selection algorithm does not have a line appearance for the line, then the call alerts on an intercom button.
When the hunt group extension that is ringing answers the call, the green LED goes steady (red off) and all other extensions in the system with the line appearance transition to the green off/steady red LED indication.
After three rings the call shall hunt to the next available extension in the hunt group using the hunt algorithm. When the call hunts the previously alerting extension stops alerting and returns to the idle condition. If the call was ringing on a line appearance, the line appearance state changes to slow red flashing indicating that the call is ringing elsewhere. If the call had been ringing on an ICOM appearance, the intercom button appearance is idled.
At any time while the call is hunting from extension to extension, any extension in the system can answer the call by either touching the line appearance of the line, or using one of the pickup features (active line pickup, call pickup, group call pickup).
An outside call that hunts never goes to voicemail and will hunt until answered or abandoned.
Outside calls ringing into a hunt group to a targeted extension are eligible for internal forwarding that might be active at the targeted hunt group extension. The call will be forwarded to another extension and if unanswered, continues hunting away from the forward-to extension to the next hunt group extension. If forwarding to an outside number is active at the targeted hunt group extension, then the call is never forwarded and alerts the target normally.
If coverage is active at the targeted hunt group extension, it is not followed and alerts the normal number of rings before hunting on to the next hunt group extension.