Hi,
It is hard to advise you because the type of telephone conversation will vary depending on the academic.
However, treat it as a formal interview and although they can not see you remember to do thing like smile. This will influence your tone of voice and also can relax you.
Why do you want to do this PhD? Why are you keen? They will want to know this. So think how you can summarize this into a sentence or two in case they ask (and of course on the phone they can't see you reading off any notes you may have prepared).
Do you already have experience directly related to computation and fluid dynamics. Talk about it positively what you have enjoyed.
Have you done other modeling that has taught you problem solving skills? You don't already have to be an expert!
Things that have gone wrong in the past are "challenges you have overcome". Things that have gone well in the past are "things you worked at to make a success".
You probably will get a question you haven't thought of because all interviews are like that: so just be honest and try and keep cheerful.
Hope it goes well for you.