Free and Open Source Software

Hey Beehaw, I wanted to check if anyone knew of any good Foss alternatives to slack?

I live in a co-op, and we currently use free slack to organize our online discussions, but we've run into issues with the free version (namely being unable to see posts older than 3 months). Paying for pro is way out of budget, so Im looking for alternatives.

We could probably self-host if required, assuming it doesn't require a ton of power. And it'd be very important for it to have a good phone app or phone front, as that's how most people interact with the internet. We're only just shy of 30 people, so no need for super-high capacity. Thank you in advance!