Depending on the exact use case, there are some other possibilities that don't run "natively" on Guix, but are still free software and could be installed from Flatpak or Nix. E.g. the Signal client for desktop and Matrix Element.
Though, actually, I think the Nheko client for Matrix (available as a Guix package) also has voice/video call support though I haven't tried it before. --Ben
