When leaving voice messages, many people make the mistake of mumbling or rushing through their delivery. Know that the way you sound is driving the impression of 'You' that you are creating in the listener. It should go without saying but, before placing that call, be clear about the key messages you want to leave the other party, should you need to.
Before you begin recording that message, inhale and speak on the exhalation. This will instantly give your voice greater resonance and authority, which will help you establish greater presence and just may increase your call-back rate!