Try this.
Create a phantom acd group then a phantom ACD pilot.
In the pilot set the path unavailable to voice mail.
Next, in the call rerouting always alternative form, find a blank line and set all to "reroute" to the phantom ACD pilot number.
Next, in the call rerouting assignment form, find the numbers you want to forward and enter the "Call Rerouting Always" index for whatever mode you need. (Day, N1, N2)
Create the mailbox to match the ACD Pilot.
That should be all you need.
Downside to this is there won't be any message waiting indication.
Ralph