In the UK at least, the POTS (Plain Old …) copper phone lines carry an electrical current as well as signals and can power the handset. There are certain guarantees about this so that in an emergency your phone will still work so you can dial 999 (our original emergency number) or 112. Our fire regulations require something like 30 minutes before things should start failing. In the real world, you get out immediately and use your mobile.
We have an emergency alarm monitoring system used by businesses. Its generally known as “Red Care” which was a brand run by BT (British Telecom). You have a small device connected to a phone line (and powered by it) and it will monitor your fire detectors and building access control systems and a 24 hour manned monitoring centre will notify you in the event of an emergency. Nowadays, these devices will use your wifi and internet connection. Sometimes: old school is best.