Two rounds of phone interviews. The first one was talking with the hiring manager. The second one was with internal client in Amazon. The first round focused more on my academic research topics, as the HM was from similar background. HM also asked several case study questions, which were simple and open. The second round also asked several case studies. But I am not sure I did well, according to the interviewer's response. But I don't know what I missed, since they were also simple questions.