Unity allows you to specify up to 9 alternate extensions for any given user. Open the Unity (SA) System Admin, goto to the subscriber page you wish to specify an alternate extension for and click on the Alternate Extensions link.
In addtion to specifying alternate extensions, you can also specify a cell phone number or home phone number, and if your phone system provides caller id, Unity will automatically log the user in to their mailbox when calling from their cell phone or home phone.