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?