Actual calls for me are not that common, and most of the time I'm in one it's either my friend calling to talk to me about something, or me deciding to call him just because I can (usually to rant over something lol). It's often just idle chatter about potentially anything though, or at least it becomes that. There might be the occasional group call from time to time, but those are so rare that I almost don't count them.
I mean, I do plenty of voice chat things, or at least participate in them (not necessarily doing any talking myself, mostly just listening) because of stuff I do in games - just not over Skype. Skype is usually reserved just for IMing.