First, **the corded phones are simple machines, powered by the signal in the phone line itself.** The phone system is basically a second set of power lines with a dedicated purpose. The sound is sent as vibrating electrical signal like a speaker or headphone cable. Other signals, like punching a number, is just a specific sound, that devices are tuned to respond to.
**As long as the phone company keeps their lines up and powered, it will keep operating**, entirely independent of the electric grid. The phone company often runs it’s own generation, since they require a fairly consistent supply and are already centralized so it’s much cheaper.
**If the power grid goes down due to supply/demand issues** (e.g. hot summer day overloaded by ACs, power plant failure, …), **the phone grid is liable to stay up** as it’s not as susceptible to this failure. If a tree falls on your neighborhood lines, however, it’ll take everything out together and you’ll be completely disconnected.
The phones in your house worked off of power supplied on the phone lines. 48V DC on hook and then uses 90VAC at 20hz to make the phone ring.
With digital phone service through a cable company, a device called an eMTA, embedded multimedia terminal adapter, that has a modem and analog converters to provide that voltage to you phones.
Fiber ONT solutions would use an ATA, analog terminal adapter, to do the same thing.
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