My T has a PhD, and I call him Dr. LastName, though I don't think I actually have to address him by name often. I wasn't sure at my first session. At the end, I had to write the check and asked how to address it (some people have business names they use, for example), and he said, "Dr. LastName", so I assumed that's what he wanted to be called. He also signs emails that way, or sometimes "Dr. L" (instead of "LastName").

Once though, he was giving an example of how I might say something, and he said, "LastName... blah-blah-blah". It sounded funny, I can't imagine just calling him by his last name (short, one syllable, like: "Smith! You're full of crap!").

I don't really care, as long as it stays consistent and I'm not getting yelled at for it!