Define absurdly expensive here. I can probably guarantee that for small to medium sized business paying Slack or Microsoft for chat software is miles cheaper than self hosting it yourself.

My Google-Fu says Slack costs $18.00 /user/mo for their Business+ subscription plan. That's still relative peanuts compared to the yearly salary, let's say 60k/year, of developer you hire to self-host and maintain an on-prem Matrix/Jitsi instance with all the equivalent bells and whistles of Slack/Teams, but guess what, even then your clients/partner will send you MS Teams invites for calls, so you still have to pay for it anyway.

Then isn't it easier if you just fork out the cash for Teams so you can focus on your product instead?