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If the ATT rep was here, tell me whether your lines are now working, and whether you can send faxes outgoing, at least?
If you cannot send, as well as cannot receive, either the line is disconnected or you have no phone service on the line leading to the fax machine. Or perhaps, if the rep bumbled about a bit, the phone line may be plugged into the PHONE or P jack on the HP, rather than the LINE or L jack, where it should be (the PHONE jack is for extension phone handsets, answering machines, and so forth.)
If you can send, but cannot receive, faxes, then it's not your lines so much as something else, and you would be well advised to call the fax machine's number, e.g. with your cell phone, and listen as to how the phone # responds.
If the HP does not respond at all, but can send faxes outbound, you need to re-enable Auto-Answering on the HP - this is often as easy as pressing the Auto-Answer button once, on the HP, to turn On the Auto-Answer light.
If the above does not help with the problem, please reply telling me the circumstances and what you observe, or asking for other details of my advice, and I will help you further toward a solution to the problem.