Thanks for taking an interest in the HP Support Forums! After reading your post I see that you are unable to setup fax on your HP Officejet 8600 printer. I would like to help you out with this issue today.
Set the fax date and time.
Set the fax volume.
Set up the fax header.
And then run fax test.
Click here and follow the document to setup the fax.
It appears i have a 4 wire telephone cord. I have had this for years and never connected a fax line. So with my first attempt as you can imagine I don't have the cord that came with the unit so off to Best buy or frys or i suppose walmart i go to see if that will solve the problem. All of the other settings were already done so i will update this once i get the correct telephone cord.