The state FTDM_CHANNEL_STATE_RINGING is not used when there is media available. We have
FTDM_CHANNEL_PROGRESS_MEDIA for that, therefore the pri_acknowledge() call should not set
the info argument to avoid sending an indication of media to the other end, as that may cause
the other end to not generate any ringing tone and at that moment we will not be generating
any ringing tone either and the caller will hear only silence