G.711 gives you the 64kbps quality you get on a channel in a PRI line.
No compression is performed.
G.729 is a well accepted codec that performs compression, and with ip
packet overhead, uses about 16 to 24 kbps (can't remember which). It
gives voice quality very close to G.711.
G.723 has a noticeable voice quality change, and is in the 6 to 8 kbps
range.
The optimal is G.729 for quality vs bandwidth issues.
There are some other considerations involved but these are the main
ones.
Ray Burkholder