Proton Meet

For the "online video call" part, plenty. For the whole integration with agenda, persistent chatroom, etc… plenty, too. Nothing is 100% a match on his own, but depending on what your users actually uses, you can basically self-host a whole "meeting/project tracking" stack on your own with nextcloud at this point.

Or if you're a little bit fancier, alternative management tools. But we did not need fancy.

Matrix, it is an open standard and the reference server implementation along with some other server implementationa and the various popular clients are open source. The Element X client and the main fork of it called schildiChat both look a bit like Microsoft Teams. But there are other clients and a company could fork an existing client and customize it to suit their needs or even create a new one if they really wanted to spend the resources.

So many.

But it's a matter of the people you conduct business with, of course. I had a therapist once that insisted on doing video calls over Google Meet. Seemed like the worst fucking idea ever and I didn't stick with them long lol

Jitsi Meet

Apparently, Discord :|

Slack, but I have heard that it has gone downhill as well.