I think if both people have skype and a phone/headset its totally free, ive had a brief look and it looks a lot like MSN. I guess it works the same way.
You can still call using skype to landlines and mobiles and it's cheaper than regular phonecalls i think.
Feel free to correct me if im wrong people!! Hmmm, might look at headsets then, just that handsets seemed to be the logical way to go, replace the normal phone with a skype phone.