Ask ½ quick mental math and ½ consulting type estimation questions. It was a phone interview. You have a very short time so all work is wasted time. I would recommend having the whole logs written out so you can extrapolate between values.

Had an initial phone interview after submitting resume online with some simple brain teasers and "behavioral" questions gauging interest in the job and in the field (i.e. what my interest is in trading, whether I am applying to other positions, etc.)