Yes I have been interviewed on Skype. I had an interview with the professor and his research group, like around 4 interviews. The questions was about my motivation to apply, how I think I may fit in the group, my research experiences and so on... Don't worry they just want to test your motivation and confidence in order to make sure that you really understand the scope of research done there. Don't worry, you may get acceptance even if you don't get an interview