Are you using an answering machine?
An answering machine should be connected directly to the port that is labeled with the
telephone icon, or it should be shared on the same telephone-line extension.
Set the device rings-to-answer setting for at least one ring more than the number of rings for
which the answering machine is set. See
Changing the rings-to-answer setting
If an answering machine is connected to the same telephone line but on a different jack (in
another room, for example), it might interfere with the device’s ability to receive faxes.
Is your fax set up correctly?