It depends on the student. As a tutor, I respect my students' wishes if they don't wanna use the web cam, but as a tutor, I would always choose to video chat with them. I think the rule of thumb is that it's the student who decides whether to use voice call or video chat, not the tutor or teacher. You wait for the student to decide, not the other way around.
In my opinion, voice call is fine if you're an advanced student but if you're a beginner, it's better to video chat so that you could see how the teacher/tutor speaks and how they would pronounce words
As a student and for language exchange I prefer to use the camera. Body language, facial expressions, and fanciful hand gestures (when you can't find the word, but can make a great puppet show with your hands) really helps a lot. I would personally never go back to just voice calls if I can help it.
All that said, I think most tutors/teachers are happy to go along with the student's preference. The student is paying after all.