I don't call back numbers that don't leave messages. In my work life, anyway. Granted, I'm not a therapist and people don't call me about suicidal ideation (usually...) but I can still see why a T wouldn't call back.

My thinking (and it's only my thinking - I admit there's a level of laziness in it and other people would choose to do differently) is that my people have to communicate with me. I will move heaven and earth for you if you reach out and actively communicate with me, but I'm not going to go chasing after you if you're ambivalent about my assistance. Ask me to call back and I absolutely will. But if you call and then decide it wasn't important enough to spend 2.7 seconds leaving a message, then I don't suppose it's important enough for me to spend 2.7 seconds calling you back.

Hypothetically speaking, anyway. I'm sure the day will come when I make an exception.