Hi Mike,
You are most correct, there are a few ways to do this.
Let's say the user that was terminated has DN 8450 but we want any calls that come through to 8450 to route to the shared mailbox @ 7005. There are a couple of ways to do this.
In CUCM go to> Feature> Voicemail> Voicemail Profile and create a new Profile called XXXX (whatever name you want) with a Description called "XXXX shared Mailbox" the Voice Mail Box Mask will be 7005> Save
Then via CUCM go to the Users Phones> under Directory Number Config for 8450> Voicemail Profile and change to XXXX (created previously in above steps)>Update>Reset
Now when a call routes via forward from 8450 to Unity Connection it will reach the "Shared" Mailbox @ 7005.
Or in Unity Connection set up Alternate Extension so that 8450 is an Alternate Extension for 7005 etc.
Shown in link below;
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/connection/8x/user_mac/guide/8xcucmacx/8xcucmac040.html#pgfId-1458710
Cheers!
Rob