What to expect from a Skype PhD interview in physics?
For a specific position, their interest will be in making a determination about your "fit" for that position. I doubt that they would reasonably expect you to be already "custom made" (bespoke) for the position, but want an idea about how quickly you will come up to speed with their main project(s). Few people are productive immediately, but it is an advantage to get someone who quickly groks the main line of inquiry. 

So, I wouldn't expect something like a final exam or a tricky gotcha question romp. But more along the lines of your intellectual curiosity and the dedication you can bring to academic pursuits. 

Your masters research is probably a plus if it gives you the background to move quickly to a new project. 

But it would be a big mistake to use this to ask about other opportunities. It wouldn't be rude, but you would be harming your own case. 

Their needs are pretty specific and I would guess they will be focused on that. 

However, note that I'm not an Austrian academic and certainly not one of the people interviewing you. So take this as general, not specific, advice. In particular, I have no way of knowing how formal it is. But use proper titles of address, I suspect.