If you set this member to a non zero value to disable DTMF tone playback for the DTMF ON event indication, you must also set this member to a non zero value to disable DTMF tone playback for the DTMF OFF event indication. The reason behind this is that internal DTMF playback tone generation ON/OFF transitions are reference counted. In order for local DTMF tone playback to be managed properly, the muting of ON and OFF transitions must be the same. If you follow this rule, local DTMF tone generation playback will function as expected.