That’s an interesting find. Yes, it does look like they support desktop Linux, on the amd64 architecture only (so it wouldn’t run on the Librem 5).
It does appear to be integrated with your regular phone number, rather than a separate number, but it is app-based, rather than using the built-in WiFi calling functionality in Android and iOS.
It seems to be a binary-only distribution. (The repository doesn’t contain sources.)
So, it’s a curiosity, but not really useful for the Librem 5.