I struggled with this a bit too. A default SharePoint contact
list only has a few fields when compared to all of the fields
available in Outlook itself. I'm using Outlook 2007, and there are
over 15 phone number fields available alone. I tried adding the
"Business 2" phone number field to my SharePoint contact list, but it
was not linked to Outlook.
After poking around a bit, I found that if, from the "List
Settings" screen, you choose "Add from existing site columns" instead
of "Create Column" the field will align to Outlook. Hope this helps.
-Adam T. Anthony
MultiProcess Computer Corporation