When you have huge traffic, and your hardware doesn't manage to process full media of all calls, you can setup collecting partial media only for a certain range of known Media IPs.
Otherwise, you may setup random or on-demand recording in the Media conf table. The table allows you to set up the SRC/DST signaling IP addresses and/or number masks to record only the calls that match these criteria.
The system will filter the signaling logs first, figure out the media IPs, and then start recording of the media stream for the configured calls in a random or next X calls mode.