Log in to the 3CX Management Console
Click on the "Advanced" tab, then select "Fax Server" from the drop down.
Default Email Address: Your fax email
Enable G.711 to T.38 Fallback: Make sure this is checked
Click "OK" at the top.
Either select your fax DIDs with the "Add a DID/DDI" button on the fax to email setup:
Then:
Or you can add them via an inbound rule.
General settings:
Route calls to settings: