Either way! Whenever I have a lesson, I message my student and ask if he or she is ready. If they say yes, I place the call. It would also be okay if the student placed the call! :)

I always politely ask if I may call the student. There's no problem if the student takes the initiative and calls me. Of course if it's a long-term student, a greeting is usually enough to show we're both online, and then one of us starts the call right away.

Hi Oliver, actually you or the teacher can call. Just click video call and wait. I really like Julio Montenegro. He is my Spanish teacher and he explains everything. http://www.italki.com/teacher/1329623 Cheers...