My t and I have the same education level so if I were to address her, it would be by first name. When I repeat a conversation, say, I've had with H about her, I use her first name in context. When I'm in the waiting room, I've noticed that her receptionist tends to refers to her on the phone with other clients as Dr. X. The few times the receptionist has called me, she refers to t by first name. My guess is that t lets clients take the lead on what to call her. I'm rambling...