I think that when set up your fax machine should recognise the incoming handshake series of strange beeps and electronic tones. Your fax should automatically send out a similar signal that lets the sender's machine know that your fax machine is indeed a fax machine before sending to you. The reverse happens when you are sending.
I don't know whether you have the set up instructions so I have posted the link. Read on from page 9 . It will open in pdf.
http://h10032.www1.hp.com/ctg/Manual/c03229151.pdf