Log in to 3CX
Navigate to the 3CX Admin Console
Advanced > Fax
Fill in Data
Default Email Address: Enter your fax email. Calls to this number will be assumed to be faxes and will automatically be converted to the PDF format, which will then be forwarded to the email specified.
Assigned DID number(s): Assign DID numbers to this fax number from the drop-down menu.
Enable G.711 to T.38 Fallback: Make sure this box is checked.