Hello,
You can do this on the CallManager several ways; with a translation pattern, a CTI route point, or a dummy phone that is set to forward all to Unity.
Make sure the number you are forwarding from in CallManager matches that CallHandler extension.
The same goes for the CallManager, either create a phone that matches that DID, or a translation pattern that matches it and then transforms it to the CallHandler number, which gets forwarded into Unity.
A couple of guides to help you out on this are here;
CallManager Dial Planing...
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a0080094b2a.shtml
How to Route Calls to Cisco Unity AutoAttendant from an IOS Gateway
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_tech_note09186a0080111bcf.shtml