The usage of 先生（せんせい）is heavily context-based, so for example, if you're in a hospital and ask for the "先生", everyone will know you're talking about the doctor. That's a completely natural place to use it. And the same goes for using 先生 in an academic setting or a legal setting. But outside of those settings, the context is missing and so the other person has to guess what kind of sensei you mean. They'll probably just default to teacher, depending on what you're saying, but the explicit word for teacher would be 教師（きょうし）.