Have you just setup a new All-in-One with all the bells and whistles? You have tested all the new features out except for one? The Fax Function? Let me guess, you can’t think of anyone to send a test fax to and back from?

Well then, I have the answer for you – HP Fax Test Service

Just Fax a basic single page document to: 1-888-473-2963 (1-888-hpfaxme) – in about 5-10 minutes later, you should get a reply fax of what you just sent.

Source: Testing Your HP Fax Machine with the HP Fax Test Service