That's not a problem. That's by design. The standard behavior for the
Outlook Address book is to display all electronic addresses (both fax and
e-mail). That behavior cannot be changed. The only way to prevent the
display of fax numbers is to store them in a different field or to disguise
them (e.g., precede them with an alpha character) so that Outlook won't
recognize them as phone numbers.
There are also a number of utilities available that can do this for you
automatically. Some are listed here:
http://www.slipstick.com/contacts/nofax.htm