You need to make settings on the printer to use an optional output tray for printing received faxes.
When using the staple finisher:
Select in the following order.
Settings > General Settings > Printer Settings > Output: Fax > Finisher tray or Output tray
If you select Auto, the printout destination is set to the output tray.
You cannot use the stapling feature for incoming fax printouts.
When using the inner tray:
Select in the following order.
Settings > General Settings > Printer Settings > Output: Fax > Inner Tray
If you select Auto, the printout destination is set to the inner tray.
When using the finisher tray, do not remove your printouts while the print job is still in progress. The print position may be misaligned.