Agreed, with one caveat.
The old Western Electric model 2500 handset of the '80s lived happily off the 48vdc line power provided by the TELCO's central office's battery power plant equipment when your house's commercial AC power went out.
But phones got fancier (speakerphones, answering machines, etc.) & they needed their own power sources which meant a transformer had to be plugged into one of your AC outlets to have a working device.
Nowadays anyone purchasing a telephone for use with a traditional landline should make sure that the device will still provide basic operation from the telephone line's voltage if commercial power is lost and if not have a dedicated UPS power back-up to supply it until the power comes back on.
At a minimum I'd have one basic line powered handset that you know will work so you can have a reliable device when the lights go out.
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