Not every VoIP device supports every protocol.  For example G.729 and G.723 are licenced, and so some manufacturers have chosen not to include it – particularly in free VoIP software including X-Lite and asterisk.

Fortunately the SIP protocol allows the two VoIP devices to negotiate which CODEC to use – for each call – at the start of each session. Therefore, the selected CODEC is the preferred CODEC and not always the used CODEC.