I wished my Organization would adopt Mattermost. Using it for years as a Chat to stay in touch with my friends. My Org uses MS Teams (because it is already bundled with MS 365). What Mattermost is missing is a Videochat / Videoconferencing option - there is a plugin but I didn´t tried it yet: https://mattermost.com/marketplace/microsoft-teams-meetings/
Mattermost v7 was released on Thursday and includes video conferencing functionality! [0] > Channels: Voice calls and screen share (beta) [0] https://mattermost.com/blog/mattermost-v7-0-is-now-available...
have you tried running it using Mattermost team (the OSS) instance?
I also use their cloud instance to stay updated on new features but voice is a paid only plan, so couldn't check it out