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'Sleep mode' would not affect faxing that way. I presume that the whole machine is not asleep since you only mention faxing problems... so please let me know what a try at making a photocopy produces... just one page is enough, even in black-and-white mode, to tell me, thanks.
As for the problem with faxing... your line may be down (exactly how depends upon what sort of phone service you have, landline/POTS, or digital/UVerse/Vonage/MagicJack service... let me know, please) or you may have your phone line connected up wrong.
The phone line cable from your phone service, must go into the L or Line jack, not the P or Phone-handset-logo jack, for things to work properly.
And the other end must go to your phone service, with no breaks on the way. Examine it, re-seat both ends of the phone cable, and also disconnect any other devices on the line. Then try again.
Note also: if you have DSL or ADSL modem on the line, then you need a 'DSL filter' dongle on the line to the fax, in order to receive and send faxes properly. Check for that, if you have DSL internet service.