Initially, I have tested every port of FXS module on ISR with an analog phone. There
was ringing tone and I was able to communicate to other as normal. The second
test involved measuring the ringing voltage on the same ports by using a pair cable and multimeter. The
'idle voltage' measured was around 44V and when there was an incoming call, the measured 'ringing
voltage' was close to 0V.
So why the ringing voltage is drop to 0V?
As mentioned in the article, is too complicated for most practical purposes.
Just verify that the interface works OK, and that's it.