``call`` commands now handle various output. Beside the original one (now named
``simple``), a new ``gui`` one display a full featured GUI (make with Qt).
PyQt 6 or more needs to be installed.
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This command has 2 subcommands: `make` and `receive` to make a new call or wait for one.
When call is in progress, a window will be created to show incoming stream and local
feedback, and a text UI is available to (un)mute audio or video, and hang up.
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