To communicate voice information, SFLphone uses some codecs. You can choose which codec to use in the audio configuration panel.

In this panel you will see the codecs available on your system with all their parameters. SFLphone will use the first activated one in the list. So you can change the order of the list, and activate/deactivate some ones, to change which one will be used. To do that, select a codec and click on the up/down arrows, or just check/uncheck the corresponding checkbox.