I used the same trick recently where a long power outage had me staying at my relatives until power was restored. My answering machine doesn't work without power but the phone does so if it doesn't pick up I know the power is out.

Since the central office switch is usually powered by generators I was used to the phone always working even during outages. But I found that the landline phone only stays functional for only a day now. I believe the deployment of fiber to the node means there are battery backups at the nodes that only last that long.