Welcome to HP Support Forums. 
I understand that you have an HP OfficeJet Pro 8720 All-in-One Printer and read that when you click on the Fax Tab in the EWS page of the printer, a message appears saying "You cannot use this function because it has been disabled. For more information, contact the person who set up the printer." A factory reset could not help. Not to worry, I'll be glad to assist you with this case.
During my research I found that we will have to perform a semi full reset on the printer. By doing this, the printer will restore to its original factory condition. All settings will get reset and it may disconnect from the wireless network (if connected). You have have to setup the printer again.
A private message has been sent with the reset instructions.
I genuinely hope the issue gets resolved without hassles and the unit works great. Let me know.
Cheers!