When making a call over the Internet, the software (soft-phone) or hardware needs to use a codec to send/receive information in a certain format and convert it to the sounds you hear.

Generally, a codec with a higher bandwidth requirements provides better voice quality (If your Internet connection is fast enough to support the codec). Most VoIP providers/hardware/licensed software will support G.711 and G.729 (However be sure to check this before purchasing hardware, or signing up with a VoIP provider!). The G.711 codec requires a connection almost 3 times faster than that required by the G.729 codec. If you are using a free soft-phone, then G.729 may not be available to you; however, the GSM codec should be, and will give you similar call quality to that of a mobile phone. During a call, the following data would be used with the G.729 codec: 31.2Kbps up and 31.2Kbps down= 62.4Kbps in total 62.4 Kbps= .0076MBps (MickJT would like to see the figures here, it is neither .0076 in IEC nor SI standards, but however is the average between the two) 60seconds x .0076= .456MB per minute. So the G.729 codec uses roughly .5MB/min during a VoIP call. The following table shows bandwidth requirements for many common codecs. Codec.................Bandwidth Usage (Up/Down) G.711 (64 Kbps).......87.2 Kbps G.729 (8 Kbps)........31.2 Kbps G.723.1 (6.3 Kbps)....21.9 Kbps G.723.1 (5.3 Kbps)....20.8 Kbps G.726 (32 Kbps).......55.2 Kbps G.726 (24 Kbps).......47.2 Kbps G.728 (16 Kbps).......31.5 Kbps GSM (7 or kbps).......low ILBC (15 Kbps)........27.7 Kbps