Hi David,

Have you looked at using this feature?

Alternate Extensions

On the Alternate Extensions page for a user account, you can enter phone numbers to set up alternate extensions in addition to the primary extension of the user. Alternate extensions can be used for various reasons, such as handling multiple line appearances on user phones. Alternate extensions can also make calling Cisco Unity Connection from an alternate device,such as a cell phone, a home phone, or a phone at another work site more convenient.

When you specify the phone number for an alternative extension, Connection handles all calls from that number in the same way that it handles calls from a primary extension (assuming that ANI or caller ID is passed along to Connection from the phone system). This means that Connection associates the alternate phone number with the user account, and when a call comes from that number, Connection will prompt the user to enter their password and log on.

From this doc;

http://www.cisco.com/en/US/products/ps6509/products_administration_guide_chapter09186a00805570b6.html#wp1040977

Hope this helps!

Rob