Brother MFC-8420 User Manual page 333

The vCard used must contain a work fax or home fax number.
When you choose work fax or home fax from the Stored Fax
Numbers pull-down list, only the fax number stored in the
Mac OS X Address Book for the selected category will be used.
If the fax number is stored in only one of the Mac OS X Address
Book categories, the number will be retrieved no matter which
category you have selected (work or home).
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