System Programming, Busy No Answer on an analog fax...

I know this can be done, though I'm not certain if "Busy No Answer" is the proper term in a case like this.

I have two fax lines, both already set up, working, and have DiD numbers programmed for them.

I know the extension number and the port number for both.

What I want is to set up a "Busy No Answer" sort of thing so if Fax number one is dialed and it is busy, it rolls over to the second fax line.

I know how to do this with actual physical telephone stations, (Speaker 43 then the extension number), but I don't know how to set up this sort of thing on a fax line that has no physical DTU 16D station associated with it, just a fax machine. I've searched the IPK programming manual (I have one) for the solution, but it's a tough search when you aren't sure of the proper terminology for what you want to do.

I am somewhat comfortable with programming the system, I know how to get into programming mode, I've programmed station numbers for new stations, DiD numbers, hold recall times etc, mostly the simple stuff.

I'm a Systems Admin and I know just enough NEC phone stuff to be dangerous, but hey, I'm learning and I have yet to trash our system. :-)

Any NEC IPK gurus out there that can help? I'm guessing this is a bit more complicated to program than I'm hoping, but  if anyone can help, I'd appreciate it.

Else I'll just end up calling our vendor and have the tech come out to do it, but honestly I'd like to learn to do it myself as much as possible.