Voice Encrytion/Scrambling

Can anyone comment on and give pointers to devices designed to
encrypt voice transmission over POTS lines?  I am interested in 
the respective sound qualities (if you've tried it) and
relative strengths of encryption algorithms.  Thus far, I've found:

http://www-jb.cs.uni-sb.de/~khuwig/lc-1eng.html
http://www.cescomm.co.nz/cesindex.html
http://www.worldaccess.com/~djm/crytek.htm

The first on the list claims to use RSA but gives no ordering
or pricing information.  Conversely, the last two seem more
"available", however they are each vague on the specifics of
their respective encryption/scrambling algorithm.  Anyone?