I recently acquired the telephone pictured below in a non-working condition. After replacing a faulty line cord and open-circuit receiver inset (originally a 2" 100Ω paper-cone speaker), everything works except for responding to a ring signal.
As can be seen from
a youtube clip I found, the LED eyes should flash and it should make a chirping sound from a piezzo-buzzer. Neither of these happen, though the eyes do light up when a loop is made. I suspect, therefore, that whatever generates the current to operate the LEDs and piezzo-buzzer isn't working. This time the circuit contains a single IC labelled "YC 2080", but I can't find any data for it.