Hi Phillip,

Hope all is well :)

The problem as you probably already know is that Unity sees the Original Redirecting (original called) number.

All pre - 5.x Unity versions use the OriginalCalledParty and OriginalCdpnRedirectReason fields in SCCP StationCallInfo message from CallManager. The feature in Unity 5.0 will allow admin to configure Unity to look at LastRedirectingParty and LastRedirectingReason fields instead, on a system-wide basis."

Seeing your reference to the System wide setting change and not wanting to go that route, here are the options;

So what you need can be accomplished in at least two ways. Lets say you want User A (Main Phone) to route to User B's (Second Phone) Voicemail Box when calls are not answered, you can configure a Voicemail profile (in CCM) for User B and then apply it to User A under Directory Number Configuration > Directory Number Settings > Voice Mail Profile

Or in Unity set up Alternate Extensions so that User A is an Alternate Extension for User B etc. Sharing a Cisco Unity Voice Mail Box between Two or More IP Phones

Configure Alternate Extensions

Open the Unity System Administrator web page.

Navigate to the subscriber's profile. Select Subscribers > Find and Select a Subscriber > Enter Subscriber Information then click Find and click the Subscriber's name for the subscriber that owns the primary phone (Phone B).

When the subscriber page comes up, select the Alternate Extensions option and click Add.

Enter the alternate extension number (in this case Phone A) and click the Save icon.

From this good Unity doc;

Sharing a Cisco Unity Voice Mail Box between Two or More IP Phones

http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_configuration_example09186a008015ceec.shtml#steps

This is an all or nothing type change Phil, and can't be done on the fly by the user :(

Hope this helps!

Rob