I saw on the local news this morning that an AT&T outage meant that people on mobile networks other than AT&T couldn't call 911 here (San Francisco Bay Area). Their suggested workaround was to make sure you have wifi calling enabled. Okay cool thanks I'll just make sure that my emergencies only happen in areas with good wifi.
The best part of it too was for Wi-Fi calling, if it was off, couldn't be turned on since it relied on AT&T's website/capabilities to approve and authenticate. That was at least my experience as well as my family's.
So if you hadn't already enabled it before the outage, you literally couldn't turn it on.
Thankfully I already had it enabled since the area in and around my house seems like a dead zone for cell phone signal. I only have one bar of signal when I'm inside.