Hi, thanks for the info link. I actually saw this infa a while ago, but it didnt give me a solution.
"For all these limitations (except for Persistent Chat), one possible workaround is for one user to start a meeting and invite the other user to join it. This workaround is the basis for interop escalation."
This workaround i tried it, but it didnt work. That's why i asked this community for a real solution.
Except that Microsoft will move your tenant across to Teams real-soon-now whether you want to or not... And yes, it's caused my main client a lot of angst and confusion. All you can do is delay the upgrade by 30 days.