Not necessarily true. One-frequency DMR conventional has two timeslots, usually one talkgroup assigned to each slot. If I recall correctly when I owned a TRX-1 encryption displays an "E" and plays a busy signal tone. Voice inversion could be a possibility here instead (or someone who really doesn't want eavesdroppers overhearing the conversation )
On some DMR encryption does not activate the tone on the TRX-1. Sometimes you get the usual garbled digifuzz. You can also change up what you hear for encryption on the TRX'es.