Hi, @Rachael Fowler
Glad to hear from you.
1.For the individual desktop client(Outlook for Windows), you cannot modify the default values of the GAL user in the desktop client yourself, i.e. you cannot add or remove fields, but you can contact the organization administrator to help you modify them so that they can be updated to the entire GAL. More information can be found Who should I contact if I need to change my information in Microsoft 365? - Microsoft Support
2.For personal web clients(OWA), you can modify the user defaults, as shown in the image below, and it's worth noting that the changes are only visible to you.
3.For administrators, you can update the entire GAL by exporting the GAL to a .csv file, modifying the details of the contact (e.g. fax) and then importing the .csv file into the GAL again. The following articles may be helpful to you.
How to export GAL (Global Address List) to CSV file in Outlook? (extendoffice.com)
Create or edit .csv files to import into Outlook - Microsoft Support
4.Also, for administrators, you can replace the labels and values of contact fields with registry keys. More information can be found Customize the profile card in Win32 apps using registry keys - Microsoft Support
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