This is called "Wi-Fi Calling" and it also seems to be down for those affected. (n =~ 150 phones I'm tracking)
Wi-Fi Calling is a mix of GRE/IPSec tunnels to carrier servers (which were still functioning during the outage) that then hook into cellular infra on the backend (which wasn't working). On my firewall, I was seeing phones make that GRE tunnel connection, but it was just a gaping void after that.
FaceTime does not use cellular infra at all (except as a potential IP network interface)