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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);

  1. G.711u
  2. G.711a
  3. G.729a

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)

Codec Bitrate/Bandwidth MOS
G.729 8kbits 3.9
G.711a/G.711u 64kbits 4.1

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