Windows Fax has some shortcomings concerning how address lists are being used. The original design was to allow the Windows Fax client to use Outlook's address lists whenever Outlook was installed. However, due to incompatibilities between 32-bit and 64-bit, there are some issues where the fax client would not use Outlook, even though Outlook has been installed properly. Unfortunately, if such is the case, there's no real solution, other than switching the architecture of the OS or upgrading to a newer OS. I've jotted down a short and simple matrix of OS and Office versions to indicate which combination is known to work properly with Outlook's address lists.