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Saurabh Misra (smisra@eos.ncsu.edu) writes:
"Horrible is not an objective description [of voice quality].  If you realize 
that the voice compression algorithms used in ZyXEL modems are meant for 
telephonic use then you will find the quality quite good.  If you are expecting 
voice quality that compares to that of a radio receiver tuned to a strong 
channel, then you will find the voice quality 'horrible'.  

Basically, the voice quality (especially ADPCM3) is very good and completely
acceptable for telephone use.  As far as the caller is concerned, he won't
know the difference between the ZyXEL's outgoing message voice quality and
that of an answering machine.  This is assuming ADPCM3.  ADPCM2 and CELP are
significantly deteriorated versions of ADPCM3."

Brent Mosbrook (brentm@zyxel.com) [ZyXEL USA] writes:
"Voice quality can vary depending on a couple factors:
1) recording format (4-bit ADPCM is best sounding)
2) recording method (remote phone, local handset, etc)

To eliminate the need to unplug your handset and phone line (current method
to record msgs locally), we have released a switch box, which will allow
you to plug in your telephone, modem, and phone line to the box, and then
hit a toggle switch to record and playback your messages through the 
TELEPHONE, rather than just the handset."

Kolja Waschk (kawk@Yo.COM) writes:
"I believe it's a problem with the quite simple coding algorithms used in the
ZyXEL."  (c.f. T.18A)

Apparently, better voice quality is forthcoming.  To improve voice quality, 
the number of bits and/or the sample rate could be increased.  (c.f. T.19B)