HP fax "device is busy" error

[Boringguy4]

I need some help, until now I've been an A3 tech, with a few A4s sprinkled in, Konica, Sharp and Xerox. Now i'm mainly HP.
I have a customer complaining that their HP PageWide 586 is giving a "device is busy" error when they try to fax. They have verified the line is good.
I had this same issue at my last job a couple months ago, it resolved itself over the weekend. i've searched and googled to no avail.
Thanks.

[Lance15]

Which revision of FutureSmart firmware is it running? FS3 or 4 (FS3 is very "busy" looking. FS4 is a clean "box type" home screen)?

Is the current revision of firmware installed? FS3 is at 3.9.5 and FS4 is at 4.9.0.1.

Are they using a POTs line or VoIP line for FAXing?