I have a FirstNet SIM for my phone (the first responder network). I've never experienced it, and it is only during designated events (i.e. a switch has to be flipped at the carrier, so might not have worked here depending on the outage), but while my phone is nominally on AT&T during said events a few things are meant[1] to happen:

Voice calls should be prioritized over other network traffic. Data should be the same. And my phone should roam from AT&T to VZW (or even TMO).

[1] having said that, I've not looked too closely, and it sounds like (unsurprisingly) that might not exactly be the reality...