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The phone ring in the U.S. is a 20Hz, 75VAC signal which is riding on -48V DC. You need to detect the AC (without drawing any significant DC current) and convert it into a higher frequency audible signal (20Hz being pretty well inaudible). The circuit to detect this must be isolated from ground. The equivalent load for 1 ringer is defined to be an 8uF cap in series with 6930 ohms. It's good practice not to exceed that load when connecting to the phone line.
I made one many years ago using a unijunction transistor oscillator powered from the ring signal to generate high frequency pulses (a kHz or so) which drove a small speaker. Made kind of a ratty sound, but you knew when the phone was ringing.