They use both AC and DC on the same phone lines for different functions. They use DC to register an off-hook condition and to power the carbon transmitters (microphones) and AC to operate the ringers. Later phones use the same DC to power the touch-tone electronic circuitry. It’s about simplicity, not stability.
AC is not on a phone line. Pulsating DC is what operates the ringers. On the older western electric phones a capacitor sat across the ringer {bell} in the phone. That was how people got busted back in the day for having an extension phone they were not paying for. The Test Board could read the induction of the capacitor. Cut that capacitor and they had no clue LOL. That doesn't matter now though. A Capacitor will pass PDC that is why they used PDC as PDC doesn't produce A/C HUM. The voltage was 96 PDC. You do not want A/C directly on phone cable.