If we could pare our use of Slack down to just text, it would be a great alternative - but until them I'm stuck using the official client.
I'd love to see some real focus on open standards and enforcement for those standards at a legislative level (the first to go should be any limits on 3rd parties acting on behalf a user... it's essentially the same as the court case that allowed consumers to install 3rd party equipment at telephone jacks)
But until then (and I'm not holding my breath because the US congress is utterly, woefully, frankly perhaps intentionally, useless for both antitrust and tech regulation at the moment) I'll continue to use the first party software.
Two years ago the multinational company I was working at had a global ransomware attack and we had a teams call for like 7 days straight with check-ins every two hours for everyone but there were on call people in there always if you had specific questions.
It worked really well and made it easy for people to get stuff done and have clear chain of command when everything else was down.
The tiny window is sorta useless except for mute/hangup, but you can undock it from the sidebar and make it much larger for video chat or screensharing.