The two main differences between voice codecs are quality and bandwidth utilization.
Wi-Manx support several codecs, although we recommend the following (in order of preference);
Each codec provides a given quality of speech and varying bandwidth utilisation. A common scoring system known as the MOS (Mean Opinion Score) attempts to give the codec a quality rating on a scale of 1 to 5. The following table shows typical bit-rates, compression delays and the equivalent MOS (Where 5 is best, 1 is poor)
There is a trade-off between voice quality and bandwidth. We recommend using G.711 as it provides the best call quality.
For further information please see the following link (Technical)
Understanding Codecs : Complexity, Hardware support, MOS and negotiation