G711

G.711 is the base codec used on PSTN.
There are two standards to this codec, u-law used in North America, and A-law used in the rest of the world.
This codec uses companding which is a form of compression, and uses 64kbps of bandwidth

G729A

G.729a is a highly compressed codec which uses a mere 8 kbps of bandwidth, but as a result requires significant computing power compared with other codecs.
In addition, this codec is also covered by various patents, so a licensing fee is applicable. Typically you pay per channel. Our undertanding is that license is not required if it is passed through however, that is to say, if you have a server that simply sits in the middle between two end points. However, bear in mind that if that server handles voicemail, it is considered an end point, and therefore would be required to have a license.
As ever, read the license carefully.
We have found the difference in call quality to be very similar to that of GSM, but with a bandwidth usage nearly half that of the 13kbps GSM Codec.

GSM

This codec is excellent for low bandwidth conditions, and one could equate it to mobile call quality without the fadeouts. We use this for call centre installations where a high number of simultaneous calls are required. One recent example allowed us to provision a 50 seat call centre making some twelve thousand calls a day on just three standard broadband lines, each line handling 15 calls each! A quote for a fixed line system came in at 42,000 pounds, with on going line rentals of £480 a month. Our solution came in at under half the initial outlay and with monthly costs of £120, which covered the three phone line rentals and broadband.
Without even considering the 60% savings on call costs, this system paid for its self within 12 months.
This is the power of Voice over IP systems, in cases like this, conventional systems can’t compete!
The GSM Codec uses 13kbps

iLBC

iLBC stands for ‘Internet Low Bitrate Codec’ This Codec provides for good sound quality in low bandwidth situations. Because of its complex algorithms, it needs a fair amount of CPU power.
While there are no royalty fees, the license requires that you notify the owners when it’s used in any commercial application. See License for full details.
Our experience has shown this Codec to have variable results. Very good quality most of the time, but when it’s bad, it’s really bad.
iLBC uses 13.3kps (30-ms frames) and 15.2 kbps (20-ms frames)