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Now, that I got some experience using video call software for calls from Germany to Vietnam:

Wire: Can't recommend. It works fine in Germany / Europa and probably for the USA. But connections to Vietnam get lost a lot and are distorted.

Nextcloud Talk: Forget it.

Jitsi Meet for Android: Works quite well with minor connectivity issues, but smartphone gets hot, drains battery quickly.

The Open Source solution that I was looking for: Not existent.