The VoIP configuration section in the Vigor VoIP routers allows you to choose a “preferred codec”.

Basically, if you don’t want to use G.729a/b codec, you simply have to choose another codec.

However, this is the “preferred” codec, not the “only” codec.  Since both VoIP devices negotiate which codec to use on a call-by-call basis, there will be some calls for which use a codec other than your “Preferred” codec.