I have several clients that have told me they ask for a message just for wanting a voice message back. Problem is - if they don't tell me this - I leave a very, very general message of "I received your call and you can call me back or we'll talk in session". However, if I get a voicemail stating - I was hoping you could just leave words of encouragement for me to make it through the week, I do so.
I think it's important to meet your needs, and to do so, you need to be very direct in what they are.