I don’t think a t has a responsibility to ring back if there is no message.
I once rang my t very early on a Saturday morning! I was looking at her new what’s app picture and pressed the wrong button.
About ten minutes later she rang back. I told her it was a mistake but if it was me and I was a therapist, I wouldn’t just ring back every person who rang and didn’t leave a message. Ts are not mind readers!