Behaving as designed. The Outlook Address Book is intended to display all
electronic addresses to which you might want to send messages. Outlook
considers fax numbers to be valid electronic addresses, since there
are many client- and server-based components that can use such addresses.

One method to hide fax numbers from the address book is to prefix the fax
number with one or more letters (maybe B for business fax, H for home, O for
other). If the fax number begins with a letter, Outlook won't show it in the
address book.

There are a couple of utilities that can help you hide fax numbers from the
address book. See:
http://www.rsoutlook.com/us/rshifa.htm
http://www.sperrysoftware.com/Outlook_Solutions.htm (Hide Fax Numbers)
http://www.mapilab.com/outlook/toolbox/

Another way to avoid having fax numbers appear in the address book is
simply not to enter data in the fax fields. Put fax numbers in some other
(non-phone) fields.