Forward on busy and FXO

I need to know if this will work (and if so, how would I get it to work).

I have one pstn line (xxx-5585), and a VoIP number as well (xxx-2222). I have an asterisk@home box with one FXO card. The system works well for the 2222 number and is not yet hooked up to the 5585 line.

If I hook up the pstn line to the FXO card, is there any way that I can get it to forward to the 2222 number all the time? Right now call forwarding is handled by my telco, and if one person has called 5585, it will forward correctly to 2222, but until that call has been ended, any new calls to 5585 will just give a busy signal.

I know there is a feature called ‘forward on busy’ (*90), but am unclear how that would work… especially for the pstn line, how would I tell asterisk to do it?

Okay, so I got a chance to try to set this up tonight. It doesn’t work.

If I dial *90, I just get a busy signal
If I dial *91 it tells me that fwd on busy is deactivated (like it should)

What do I need to do to get fwd on busy to work?