I've tried everything I can think of on this, so now I'm reaching out to
you good people.  I have a FreeSWITCH system that has the latest version of
FreeTDM, Dahdi, libpri, and a Rhino R4T1 quad T1 board with up-to-date
firmware.  When I call into one of the PRI channels, a "playback"-played
file sounds clean, but when I go into conference, the conference audio is
distorted.  This does not happen with a SIP channel.  I would classify the
distortion as a "crackle", but it is very regularly periodic.  Repeating
ticks at (roughly) 10-20 Hz repeat rate, I would say.  I can only hear them
when audio is present, i.e. the audio seems to be modulated with the ticks.
 And that conference audio can be the MOH guitar recording.

I've eliminated the following:

- T1 clocking
- Echo Cancellers (I've turned on and off all combinations of hardware and
software EC)
- T1 board firmware (it's the latest, and the problem only happens
on-conference).
- Packet length - it's the same (20 mS) for the conference engine, FreeTDM,
and Dahdi
- mu-law vs. a-law encoding - behaves identically.
- Comfort noise - happens with and without.
- conference sampling rate (tried 8000 and 16000, even though I'm using
TDM, which is 8000).
- FreeSWITCH version - happens on 1.2 stable, the latest master branch, and
an older version that we were using.

If you haven't heard of anything like this, could you please give me your
thoughts on why this could happen only on-conference, not while playing WAV
files, and only with FreeTDM channels?

    Thanks!
    Bob