Things can change if you are talking to professional teachers, tutors or language exchange partners.
With teachers or tutors the first call is used to assess your current language level and to know your needs and goals.
On subsequent calls you can rely on them to conduct the conversation, but don't forget to give them a feedback to help them to be effective.
With language exchange partners, and, sometimes, with tutors, you can set a subject for the current or next call, or just have smalltalk.
You could even exchange links to online news articles, to prepare in advance a subject for the next call.