The simplest phone you can attach to any POTS line in the US is the touch-tone phone. [0] It's a microphone, speaker, ringer, switch, and a DTMF tone generator. The most complicated part of this device by far is the tone generator. The line it's attached to provides the power for all of the electronics/electromechanics inside the phone... and is also responsible for activating the phone's ringer and "knowing" the status of the "on hook" switch. The most basic phone models have no memory or logic inside them of any kind.

Given these restrictions, how does one ensure that one can activate the ringer of a single phone (and connect its speaker and mic to that of the caller, and noone else) in a world where all of the human operators were replaced by electromechanical ones, which were then replaced by fully computerized ones? Once one has figured that out, how does one ensure accurate and correct determination of the calling parties, the transit networks, and the duration of the call? One needs to recover your costs, and one uses usage-based billing to do so. [1]

In order to do those things, mightn't the system that that phone is connected to have to have all of the information about the callers, the systems the call flows through, the duration of the call, etc, etc, etc?

[0] Rotary phones are even simpler than touch-tone phones because they replace the tone generator with an elecromechanical gizmo that bangs on the line when it's rotated. Because I vaguely remember hearing that some phone networks were phasing out support for rotary phones, I'm assuming that you're not guaranteed to be able to attach one and have it function.

[1] I'll only briefly mention POTS features from ~35 years ago such as "Caller ID", "Read to me out loud the phone number of my most recent caller", and "Keep calling this number for the next half hour and ring me if they pick up", which had to (and did) work with these dumb-as-bricks phones.