>The Ring signal is AC at 17 .. 25 Hz and the average of a half wave
>rectified signal at 5V will be about 2.5V.

IMHO: No.
It's because the opto's output / the transistor is switched with this
low frequency, thus giving an average of 2.5V ...

Take a voltmeter & you get at the transistors output:
  a) needle type   -->  2.5V reading
  b) digital type  --> 'fluctuating' reading

  c) take an osci. -->  you'll see the ring signal at full (or half )
                        frequency  _but_  'clipped' to 5V peak-peak !!


>You can place a cap across the photo transistor to hold the level
>for the off half cycle or place a bridge rectifier BETWEEN the ring
>capacitor and the optoisolator.


A capac. at the opto's output(!) will not be the solution (in a general
application), so you have to check at this point of the circuitry for
the correct frequ. range (!) of the ring signal.


>Cheers

 .. to you too

 Ralf.