It may well work differently for doctors or psych professionals, but my mother's an ER nurse in a large American city, and the way it works for her was that she can tell outsiders details (so we got disgusting stories over dinner on the regular), or she could say names. She couldn't, however, do both. So if John Doe, Famous Actor, is in a hospital after a car accident, you can call the hospital and say "Is John Doe a patient there?" and they can answer yes or no, but if you say "What are John Doe's injuries?" they can't tell you that.

The closest she got to violating this was the time she treated the victim of a crime that made global news, and she had to get it off her chest because the details were so disturbing. Even then, she didn't say the person's name, it was just that the circumstances of the case were such, and the story was so widely publicized, that there was really no question who it was.

The case anon describes above, of a doctor sharing details of a patient who just called, does seem a bit shadier -- if the doctor took the call in the same room and you recognized the voice on the other end of the line, for instance -- but in general, my understanding is that it's ok for a doctor to share, in broad strokes, information about one patient with another so long as there's nothing that can identify them.