Teams alternative without cloud connectivity for on-prem hosting
The requirement "completely without cloud connectivity" effectively excludes Microsoft Teams because Microsoft Teams is designed as a SaaS/cloud service and does not provide an on-premises version. This shifts the problem from "which tool is similar?" to "which system can be operated technically, organizationally and regulatory in such a way that no third-party cloud services (incl. push gateways, telemetry/registration, marketplace access, update dependencies) are necessary in productive operation".
