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. This definitely works in Outlook 2000 and 2002.

At http://www.sperrysoftware.com/Outlook_Solutions.htm, you'll find a Hide Fax Numbers utility that adds a text prefix to fax numbers for all your contacts and removes it when you really do need to use a fax number in an Outlook message.

Another way to avoid 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.

Microsoft has a Knowledgebase article at http://support.microsoft.com/?kbid=305361 that explains further. Note that this is behavior is normal for Outlook 97 and for Outlook 98 and 2000 in Corporate/Workgroup mode. It's a big change, though for users upgrading from Outlook 98 and 2000 in Internet Mail ONly mode.