Designing a PSTN/POTS ringing detection for uC? (AC-DC PSU with wide op. range)

[thejoggingmat]

How can I design a reliable POTS/PSTN ring detection circuit?

I tried designing a capacitive dropper hooked up to an optocopuler based on this datasheet (http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf, schematic as per page 2 fig 1 with different component values) and while it works at the nominal AC ring voltages (as per the specifications published by the governing body), it's rather unreliable at the lower end or upper end of the voltage range. Also, when the phone is off-hook, it sometimes triggers the circuit as well.

There is an enormous voltage range that is needed to work on. Requirements:
-Vmax, rms: 100V
-Vmin, rms: 30V
-Vnominal, rms: 70V
-Freq: 24Hz, +/- 2Hz

I'm using PC817 optos with appropriate resistors.

[Len]

A quick google shows lots of ring detector circuits, for example:
http://electronics.stackexchange.com/questions/52835/switch-small-dc-voltage-with-90v-30hz-ac-signal
https://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k8086.pdf
(I have not tried any of these.)

Many of them have a pair of zener diodes to limit the AC voltage, maybe that's the trick you're missing.