BigBlueButton is an open source web conferencing system designed for online learning. BigBlueButton is a tool used by instructors, which helps them access to Learning Management Systems, engagement tools and analytics.
You can install BigBlueButton on your own server since it is open-source. There are definitely no ads, but they implemented this “Learning Analytics Dashboard” that is about analyzing the activity from the participants. That tool is used in schools and not for profit purposes I guess. Either way it is a trade-free video conferencing software.