What to say when I contact my future advisor?
It's not clear why you need to send an email, if you and her already agreed to talk over the phone. Anyway, I guess you can send an email as a reminder and to set up an agenda for the phone call. 

I would just say something like:


  Dear NAME/TITLE/WHATEVER_IS_APPROPRIATE_IN_YOUR_CULTURE,
  
  I'm looking forward to our phone call on DATE at HOUR. 
  
  Just so that we have a starting point for our conversation, I'd like to please get your advice on the courses I should take (the deadline for course selection is approaching soon). 
  
  Also, I haven't heard any final word about my application from the department since DATE. I know you just got back from your research expedition, but I was wondering whether you have any update about the status of my application. Thank you in advance.
  
  Looking forward to talking to you soon,
  
  NAME