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The Auto Answer light, in many HP printer models, must be On, before auto-fax-reception will work. If there is an Auto Answer or Auto Receive light on your printer's controls and indicators panel, that is.
The Auto Answer light in this model (looking it up ...) is next to the Auto Answer button. Press the Auto Answer button once to turn Auto Answer On.... if it is Off.
HP Solution Center - Settings - More Fax Settings
has the HP Fax Setup Wizard in it, which supposedly will tell the printer (if you say there is not voice phone on the line) to set the Auto Answer to On permanently. Sometimes it shuts itself Off otherwise.