[lozf]

Both Matrix and Signal use WebRTC for VoIP so the content is encrypted by default. Call set up and signaling is also encrypted by default with Signal, and possible with Matrix - it's automatic if the room from which a call is established is already encrypted.

I know Signal attempts to prevent any data leakage by forcing the Opus codec to use a constant bitrate instead of its default VBR -- I'm not sure if Matrix implements anything similar yet.