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

The way that Outlook works is that it shows both the email and the fax
addresses in your contact list. Therefore if you have a contact with both
entries completed then it will appear more than once in you contact list
when addressing.

I you do not wish to display the fax entry when addressing then prefix the
fax number with a non-numerical character (e.g. FAX: ) and this will prevent
the fax entry from displaying but still visible when you view the Contact
item.

If you have a number of these to change or wish them to be managed
automatically when you create a new contact then have a look at WS:Fax
Display Manager from Websetters (www.websetters.co.uk) to manage for you.

Regards
Graeme