@kaia There is honestly nothing good at the moment. You can use Matrix, but good luck with calls. There is also Mattermost, which you can selfhost but I don't think it's really open source. Oh and Nextcloud has something like that integrated, but it seems pretty limited.
@fell thank you! I used Nextcloud for a while, but it had too many bugs to be usable
@kaia When the new Element Call thing is finally production ready, then I would recommend Matrix. Calls are a buggy and confusing mess at the moment, but I think they might have it figured out some time next year. The new solution appears to be much more reliable.
@m0xee @kaia Even with 1:1 calls, it can happen that two client apps are incompatible if someone is using Element X for example and the other person is using Element Web.