At first, I wrote "Element/Matrix" and decided to not be too pedantic... But if you want to be complete: the messaging protocol is, of course, Matrix. You could say there is actually no such thing as a Matrix server either, because it's a protocol. The server must probably be Synapse-based, I guess. But there is an "Element-based server" in the sense that the web interface of Tchap (and phone apps) are very clearly forked from Element, which is what I meant.

Visio is based on LiveKit, which Element Call is also based on (as far as I understand). It lives outside of Tchap. The DINUM never mentioned it was based on Element Call. Do you have additional information? (Not that the difference matters much I think)

It is always either that the license is not permissive enough, too many changes are required that upstream wouldn't merge and its would be hard to keep rebased, showing the world that they can be absolutely independent, or not having enough experience and experts for given stack. Maybe I missed some reasons, but that'd be all I think of.

Either way, if it's open sourced our company might switch.

There must be a reason for them turning to something else than

You may be new to modern software development. Switching proverbial horses is massively common, usually for no benefit (or to lock people in). It's everywhere, and especially in the corps who want that lock-in (ohai apple).

For a 2 week overlap, people on gtalk, Facebook and a regular jabber server could chat with one another as easy as addressing an email message. Then both Facebook and Google switched to their own homebrew replacement and made up some compelling-sounding, feature-laden but implausible reason. Gtalk has never sucked less than those two weeks with a working discreet client app and interoperability, though. It was actually adequate.