I've been looking for a way of doing this and can't find anything. There used to be a utility called Skype Call Recorder, but since Skype came out of Linux beta it no longer works [that means all those articles telling you to use it up until mid 2017 can be ignored].
I'm posting this on the offchance that someone knows something about how to do this - to record voice calls in Skype of Facebook - but apparently it can't be done in Linux at all. Maybe in a VM but I don't own a copy of Windows.