Everyone is familiar with the way telephone calls sound. That particular ‘sound’ is created by significantly limiting the range of frequencies that are carried through the phone network. A full range of sound, like you might hear from a CD, is approx. 20hz to 20khz. It’s very likely that the sound system that you are listening to that CD through is not capable of reproducing that entire range, but that’s Ok since only young children can hear it anyway.
Almost every phone call you’ve ever heard narrowed that 20khz range down to only 8khz, less than 1/2 what we normally hear. The brainiacs over at the open source PBX project FreeSWITCH have created a new codec that doubles that range to 16khz. The result is a much more natual sounding voice.