The interview process is very different for GTA students than for other students. The candidates from toronto invited for an interview will be the subject of a 3-4 hours group interview with various probability/math/programming questions as well as a presentation on a given topic. The ones that are schedules for phone interviews will be asked basic questions and will be more qualitative than quantitative. Feel free to ask me any questions, i am currently studying there.
Also, I would like to add that this year, the process was a bit delayed. Answers should start coming out at the end of march.