Mine goes by her first name (signs emails by first name, says only her first name in voice messages sometimes), but I avoid calling her anything directly.

Not sure why this is, it just seems awkward. Maybe something about using their name makes you focus on them as a separate person, and gets in the way of the transference relationship where you're trying to project other things on them, and that's why it feels awkward? Or maybe it has something to do with wanting to maintain a certain power hierarchy, and using her name seems too forward? Or maybe it feels too familiar to use their first name (like you're actually friends) and so I avoid it to maintain boundaries? I don't really know enough about the therapy process to understand this, but I would be curious to hear if anyone else has thought about why this happens. I've seen other threads about this, so it seems to come up for a lot of people.