Usually there is also two zener diodes (usually 10-20V models) to make sure that the ring detection circuit does not detect too small AC signals in the line as ring signal. In the picture below you see a very typical ring detector circuit for modems. The circuit just gives the idea how modem ring detector circuit work. The actual component component values selection must be so that the circuit meets the national telephone regulations (this can be usually easily done by using suitable zener diodes and maybe chancing the resistor value a little).