If you are using the DEDICATED MAP to configure the E&M to the T1, then there is no automatic failover. If the ATLAS is configured so that you dial an extension on the other side, and this is set up in the ATLAS' DIAL PLAN, then you can configure the Network Term T1s all to have the same IN#ACCEPT and the ATLAS will select the first available T1. If the T1 is down, it will select the next one that is up.
Chances are this is a DEDICATED MAP configuration, though, and for that you have to either manually change the active map, or set it up to change automatically based on the time of day.
Yes, dedicated mapping is the way it is configured.
These lines are active and carrying audio at all times.
I am not versed in the nuances of dial plans. However, I was hoping there might be a way to establish something like a permanent dial plan where no dialing actually happens, but I don't know if this is even possible.
My other nebulous idea is to operate with some sort of manually controlled physical relay on the T1 line at the local end. Setting aside out-of-spec line construction concerns, I wonder if there would be any way to get signal from either T1 at the remote end. I don't know how I would control a relay there.
Plan C is to establish two sets of E&M ports for each line in parallel, mapped to alternate T1 lines, and a gang of relays at the local end. This is actually a well established practice, but takes the most physical hardware and wiring.
I'm afraid that unless you're actually using the E and M leads, there's no way to set it up in the DIAL PLAN.
I'm not familiar with a method for plan B.
Plan C seems the best option, and we've had a few customers do this - where they have more than one E&M port connected to their communication system so audio will go over whichever T1 is up (or even over more than one T1 in parallel.