The MOS (Mean Opinion Score) scale is: 5=Excellent; 4=Good; 3=Fair; 2=Poor; 1=Bad.
Some values, taken from "Carrier Grade Voice over IP" by Daniel Collins:
G.711 4.3 G.729 4.0 G.729AB 3.9 GSM(full rate) 3.7
The above scores assume no packet loss, minimal delay, no echo.
However, IMO such scores are generally only useful for choosing among compression codecs.
If you have plenty of bandwidth and minimal packet loss, you should use G.711, not only for better quality, but because it avoids issues with conferencing, DTMF relay, etc. Also, if your ITSP has upstream routes that use a different compression scheme, G.711 avoids cascaded codecs, which sound really awful, MOS < 3 for sure.
If you don't have enough bandwidth to handle the desired number of simultaneous calls with G.711, you obviously need to use compression; IMHO G.729 is a good choice.
If you have >1% packet loss (or packets effectively lost due to excessive jitter), then G.729 may actually sound better. Lost G.711 samples are replaced with silence, sometimes with pops at the transitions. OTOH, most G.729 implementations have "packet loss concealment", which continues the previous sound, gradually fading out. With 5% loss, a good G.729 system sounds like a mediocre cellular call, but G.711 sounds terrible.
There are systems that use G.711 when traffic is light, but switch to compression codecs under heavy traffic to conserve bandwidth. I don't know how/if this can be done in Asterisk.
--Stewart
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