Sounds like something might be going on with the hardware, but let's find out for sure...
1. double-check the settings and connections: http://support.hp.com/us-en/document/c03415592
2. update the firmware: http://support.hp.com/us-en/document/c03312867
3. test the fax: http://support.hp.com/us-en/document/c00259105
If those steps fail, it's hardware, and you'll need to possible reset the printer. You can contact HP support in North America at 1-800-474-6836 or find the local telephone number for your country here: http://welcome.hp.com/country/w1/en/support.html Please look for the category "Technical Support After you Buy"
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