According to my adaptor (Linksys) the following apply:

Best (G.711u Require 64k Bandwidth)
Best (G.711a Require 64k Bandwidth)
Good (G.726-40 Require 40k Bandwidth)
Good (G.726-32 Require 32k Bandwidth
Good (G.729 Require 8k Bandwidth
Normal (G.726-24 Require 24k Bandwidth)
Normal (G.726-16 Require 16k Bandwidth
Normal (G.723 Require 7k Bandwidth)

Hope that clarifies things.