I call my T (a PhD) by her first name (although for a long time I just didn't call her anything lol...but I've been seeing her so long -- on and off for almost a decade, and we're at the point where I can text her...in fact, she asked me to text her today, so I feel first name is appropriate).

I call my pdoc Dr. LastName. It would be completely and utterly weird to call her anything else because she is just very formal. Even when we talk about my T she'll say Dr. LastName and then I'll say it as well to match her, even though that's not what I actually call my T myself.