Huge number of landlines now go through SIP tunnels over the NBN… so if your internet is down, no phone calls, not even emergency unless you have purposefully designed your network with failover to another provider. the few remaining physical landline connections become data at some point now too, so if that backend goes down, not even the emergency calls will go through.
Many of the consumer routers supplied by the ISPs now have failover to a 4g/5g connection, but it’s always the same network of course, so the failover is useless unless there’s some actual work done by the providers in allowing emergency failover to their competitors.