Capacitors block DC voltages and pass AC voltages. The AC signal that gets through will be added to the Vcc/2 voltage produced by the voltage divider, and can be read out by analogRead(A0).
Expect the output of the analogRead(A0) command to produce a time varying signal when the "ring tone" is present.
No, you do not need small signal diodes.
Edit: A safer alternative is to use a transistor audio output transformer, which you can buy or get out of an old transistor radio, or perhaps from a toy with a speaker. The transformer would also increase the voltage, so I've added an additional resistor (R3) to protect the analog input from damage.
