Codecs are algorithms especially designed to compress voice data. For example, digitalised voice in 16bit / 8000 Hz represents a data flow of 128 kbits/second. Using the GSM vocoder, this flow is reduced to 13 kbits/second, without significant loss in quality.
Linphone can use several codecs. Use buttons on the bottom of the codec list to put them in an order of preferrence. Note, that according to
your network connection type (given in the network section), some codecs are not usable. Them will appear in red and they are not selectable.
You can decide to use or not a usable codec (in blue) by changing its status with the enable/disable buttons on the bottom of the list.