You might want to try BigBlueButton as well. There is a blog post from Mattermost about it here : https://mattermost.com/blog/bigbluebutton-integration. If you’re interested in trying it out you can also head to the Github page for instructions on how to setup and integrate them : https://github.com/blindsidenetworks/mattermost-plugin-bigbluebutton#installation-and-setup