Elastix has been abandoned by the authors and the company that bought it is not interested in supporting it, so the first recommendation is always “Upgrade to pure FreePBX”.
Having said that, there are settings in recent versions of FreePBX that allow you to specify the destination of a call on any of these states. In fact, there are three choices for failure state so you can customize the callers experience. If these options aren’t available to you in Elastix, you may need to upgrade to a more recent version of FreePBX (see answer 1).
I would make the effort and upgrade. You can check here on the forum how and if those cards are supported on a FreePBX Distro install.
Elastix is as dead as it can be. No point sticking to it in my opinion, as sooner or later you would want support again, but you won’t get any.
They should work the same as they always have. Elastix is based on Asterisk and FreePBX, so there should be an upgrade path. Remember, DAHDI is a separate and distinct piece of the system that is managed and developed by a completely different group.
All of this is free software, so pulling the working hard drive, dropping in a new one and installing over the weekend might be an interesting exercise for you.