Microsoft Outlook considers fax numbers to be valid e-mail addresses. If you want to be able to send both e-mails and faxes from Outlook, there's no solution to this issue.

If you are keeping the fax numbers just as a matter of record, though, and don't use them in Microsoft Outlook, then you can choose one of the non-fax phone number fields to store the fax number information. Unfortunately, you can't change the names of the built-in fields; you would need to remember which field contains fax numbers.

Another approach is to put text in front of the fax number, such as "W: " for a work fax or just "Fax:"; this causes the number to disappear from the Outlook Address Book, but keeps it easy to see the number in the contact record or Contacts folder. The tools listed below add a prefix automatically and remove it when you need to dial the number.

Macro to prefix Fax numbers

You can run the following macro to refix fax numbers with Fax: so they won't show in the address book.

Replace BusinessFaxNumber with HomeFaxNumber or OtherFaxNumber if you need to hide other fax numbers.

Removing the Fax prefix from numbers requires more complicated VBA code to strip only the prefix and is more prone to error. Because of this we recommend using an addin that can toggle the prefix off and on.

Public Sub HideFaxNumbers()
    Dim objOL As Outlook.Application
    Dim objNS As Outlook.NameSpace
    Dim objContact As Outlook.ContactItem
    Dim objItems As Outlook.Items
    Dim objContactsFolder As Outlook.MAPIFolder
    Dim obj As Object
    Dim olfax As String

    On Error Resume Next

    Set objOL = CreateObject("Outlook.Application")
    Set objNS = objOL.GetNamespace("MAPI")
    Set objContactsFolder = objNS.GetDefaultFolder(olFolderContacts)
    Set objItems = objContactsFolder.Items

    For Each obj In objItems
        'Test for contact and not distribution list
        If obj.Class = olContact Then
            Set objContact = obj

     With objContact

          If .BusinessFaxNumber <> "" Then
               olfax = .BusinessFaxNumber
           .BusinessFaxNumber = "Fax: " & olfax 
        End If
          .Save

     End With
        End If

     Err.Clear
    Next

    Set objOL = Nothing
    Set objNS = Nothing
    Set obj = Nothing
    Set objContact = Nothing
    Set objItems = Nothing
    Set objContactsFolder = Nothing
End Sub

Remove Fax: Prefix

To remove Fax: from the Fax number, replace the If block with this.

          If InStr(.BusinessFaxNumber, "Fax: ") Then
            .BusinessFaxNumber = Replace(.BusinessFaxNumber, "Fax: ", "")
        End If
          .Save

If you have duplicate fax prefixes (caused by running the macro more than once), or to prevent duplicates when you update contacts, use this to remove and replace the prefix:

          If InStr(.BusinessFaxNumber, "Fax: ") Then
            .BusinessFaxNumber = Replace(.BusinessFaxNumber, "Fax: ", "")
        End If
          .Save
          
           If .BusinessFaxNumber <> "" Then
               olfax = .BusinessFaxNumber
           .BusinessFaxNumber = "Fax: " & olfax
        End If
          .Save

How to use the HideFaxNumbers Macro Tutorial

How to use macros

First: You will need macro security set to low during testing.

To check your macro security in Outlook 2010 or 2013, go to File, Options, Trust Center and open Trust Center Settings, and change the Macro Settings. In Outlook 2007 and older, it’s at Tools, Macro Security.

After you test the macro and see that it works, you can either leave macro security set to low or sign the macro.

Open the VBA Editor by pressing Alt+F11 on your keyboard.

To put the code in a module:

1. Right click on Project1 and choose Insert > Module
2. Copy and paste the macro into the new module.

More information as well as screenshots are at How to use the VBA Editor