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Bandwidth fact: If you don't consider your bandwith requirements when building a VoIP infrastructure, you'll wind up with an unreliable system and an overtaxed WAN. Have a look at the audio compression algorithms and WAN topologies below to help you effectively determine your bandwidth needs.

Narrowband VoIP codecs

Codec	Payload Bandwidth	MOS	Description
G.711	64 Kbps	4.1	This is the most universally supported codec used in IP telephony. This narrowband codec supports frequencies in the 300 to 3,400 hertz range and is uncompressed. Although the quality is very good, it consumes a lot of bandwidth.
G.729	8 Kbps	3.92	This is the second most supported codec and offers nearly the same quality as G.711. The key advantage is that it is compressed eight times smaller than G.711 while sounding almost as good.

Wideband VoIP codecs

And second, wideband for a rich VoIP experience:

Codec	Payload Bandwidth	Description
G.722	48 to 64 kbps	This is the most common wideband codec available in IP phones, though wideband support is only recently gaining momentum. The quality is excellent at twice the sampling rate of standard G.711, but the compression isn’t that great. But considering the fact that it’s the same bit rate as narrowband G.711 but delivers much more realistic sound, G.722 will be one of the best codecs to use if your IP telephones will support it. Wideband supports frequencies of 50 to 7,000 hertz. There are no longer any patents covering G.722, so it’s free for anyone to use.
G.722.1	16 to 32 kbps	This is a wideband codec, developed by Polycom. Its key advantage is that it’s a computationally efficient and compact codec at 16 to 32 kbps, which is less than half the bandwidth required by G.722. 16 kbps mode isn’t appropriate for noisy audio input or if music is mixed in, since the compression artifacts are noticeable. 32 kbps is good for any kind of workload. This codec must be licensed from Polycom, and it’s currently used only in Polycom’s high-end video conferencing systems under the marketing name “Ultimate HD.” Current Polycom IP phones use the marketing term “HD Voice,” which supports only generic G.722 for its wideband codec, although future IP phone models may support G.722.1.
G.722.2	6.6 to 23.85 kbps	Also known as AMR-WB, this is a wideband codec. A 6.6 kbps mode is also supported, but 12.75 is the practical bit rate for speech in a clean environment. The higher 23.85 bit rate is better for noisy conditions and music. At the time of this writing, I’m not aware of any IP phones that support this codec. It’s currently used by T-Mobile in Germany for cell phone applications.

Conclusion

The bottom line is that G.711 and G.729 will be the lowest common denominators that are universally supported. G.729 makes the most sense in the narrowband range because it sounds almost as good as uncompressed G.711 but is eight times smaller. Source: George Ou, Tech Republic