Providing users the ability to create a personal address book is a longer term goal.
We do have a feature coming soon which essentially serves as a global address book for business users but also provides agents and contact center users with additional customer context. This feature includes the standard permission control, so you do have the flexibility to make the contacts accessible company-wide or restrict view or edit rights to a subset of users.
You can store a fax number for each of these contacts and if desirable, we can add the ability to easily search for one of these contacts when you're faxing documents.
Here are more details, let me know if you have any questions or if you see anything else that you'd like to see added.
https://help.mypurecloud.com/articles/about-external-contacts/