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