By default Sent Faxes and Received Faxes objects are related to the standard objects: Account, Contact, Opportunity and Case. These are the only related objects to the faxes out of the box. To relate the fax objects to other objects you need to add a lookup field on the Sent Faxes and Received Faxes object to the object you want to relate to.

  1. Go to Setup, Create, Objects, Sent Fax, then click “New” button under Custom Fields & Relationships section.
  2. Select the field type “Lookup”.  Then select the object you want to Relate to on step 2.
  3. A default field label will be created from the name of the object.  You can override if you want.  Click Next.
  4. Set field level security for which profiles to see the field, click Next.
  5. On Step 5 indicate to add the field to the layout on the Sent Fax Layout.  Click Next.
  6. On Step 6 indicate to add the related list on the related object to see the related list of Sent Faxes there.  Click Save.
  7. Do the same on the Received Fax object.
  8. Edit the page layout on the object by referring to the following article: Modify Page Layouts