How To Make Free Skype Calls On A Mobile Phone?

There is that guy who travels a lot and communicates heavily (and freely) through Skype, and finds it just too bad to have to spend that much money on roaming mobile communication. I mentioned the many mobile VoIP solutions that would allow him to make free calls, but he rightly underlined the fact that since so many of his contacts use Skype, he would barely make any savings on other services, since it is only calls between users of a same service that are free. So here is what I could find for him.