To send a fax directly from LibreOffice, you need a fax modem and a fax driver that allows applications to communicate with the fax modem.
Open the Print dialog by choosing File - Print and select the fax driver in the Name list box.
Clicking OK opens the dialog for your fax driver, where you can select the fax recipient.
You can configure LibreOffice so that a single click on an icon automatically sends the current document as a fax:
Choose LibreOffice - PreferencesTools - Options - LibreOffice Writer - Print.
Select the fax driver from the Fax list box and click OK.
Click the arrow icon at the end of the Standard bar. In the drop-down menu, choose Customize.
The Toolbars tab page of the Customize dialog appears.
Click Add Commands.
Select the "Documents" category, then select the "Send Default Fax" command.
Click Add and then Close.
On the Toolbars tab page, click the down arrow button to position the new icon where you want it. Click OK.
Your Standard bar now has a new icon to send the current document as a fax.