Mind you that they keep paywalling previously free features. Example of them paywalling group calls and their Playbooks feature (v1 - generally available, v2 - in paid plans only): https://docs.mattermost.com/product-overview/mattermost-v10-changelog.html

Nextcloud Talk - actually free and fully-fledged unlike the alternatives that either require you to pay money or don’t support more advanced features like video calls