CODEC

For more efficient of the bandwidth in VoIP, the voice data can be compressed so that it require less space and will certainly record only a limited frequency range. There are many ways to compress audio, the algorithm for which is referred to as a compressor/de-compressor (CODEC). Many a number of CODECs exist depending on the application (e.g., converssations, music, movies and sound recordings) . The CODECs are optimized for compressing voice, which significantly reduce the bandwidth used compared to an uncompressed audio stream. Speech CODECs are optimized to improve spoken words at the expense of sounds outside the frequency range of human speech. Recorded music and other sounds do not generally sound very good when passed through a speech CODEC.

A complete list of Codecs recommended by ITU can be found here.

CODEC Resources:

VOIP Codec Tutorial (pdf)

Codec info