Instead of Microsoft Teams

Chat, calls and meetings bundled with Microsoft 365.

Partly

The chat part is replaceable. The part where Outlook invites turn into meetings with dial-in numbers is not.

You lose: Calendar integration that just works, and the phone system.

Start here

Mattermost

AGPL-3.0 (open core)an eveningactive

No native calendar or PSTN bridge; you bolt those on.

github.com/mattermost/mattermost mattermost.com

A compose file, a domain and a reverse proxy.

Also worth a look

ElementAGPL-3.0a weekendactive
Video is Element Call, which is young. Federation adds moving parts.
github.com/element-hq/element-web element.io
Jitsi MeetApache-2.0an eveningactive
Meetings only. Chat and file sharing are minimal.
github.com/jitsi/jitsi-meet jitsi.org/meet

Facts checked 8 November 2026. Licences, activity and archive status come from each project's own repository; the verdict and the cost lines are the desk's.