for which office/location are you applying? Also Deloitte oder Deloitte Monitor? First interview?
It varies quite a bit ...some do take-home cases (which are written of course), some do short cases during a call. If its the "first round" its usually about 45 Min of which 15 min are dedicated to CV/questions and the rest to a short test, interviewer led. As its on the phone don't expect it to be too quantitative; rather high level qualitative.
Let me know if you would like to prepare for this or have any other questions.