Perform a Cold Reset on your printer. This will wipe out your current fax settings as well as the printers memory incase there is a bad fax document stuck in there.
Once you are reset then reconfigure your fax machine and make sure to disable a feature called ECM (Error Correction Mode). ECM normally gets in the way with most faxing issues and should be disabled as soon as there is a problem in a fax enviornment.
Edit - Cold reset steps are here: http://h30499.www3.hp.com/t5/Printers-LaserJet/Laserjet-3015x-Changing-IP-address-or-doing-a-cold-reset/td-p/1125809#.U9ZOM9KKCjI