I'm guessing the reasoning behind this is that phone based Skype calls actually cost Skype money because more data has to be routed via them rather than the pee

I'm guessing the reasoning behind this is that phone based Skype calls actually cost Skype money because more data has to be routed via them rather than the peer-to-peer infrastructure that regular desktop Skype uses. If there's a technical reason why this is the case (i.e. you can't use peer-to-peer on phones because of bandwidth issues) then I can't see how a competitor would be able to undercut them without finding someone to subsidise it.