Mckinsey Phone call-an interview?? or screen??

Hello Everyone, Got a call from McKinsey last week from a Recruiting Officer. She fixed an appointment for another day. At the second call, we got to talk for 45 mins, really getting deep information on what I had been doing all along (basically what was in my resume) and asking me scenario questions. She concluded by asking me if I was interested in other offices etc She also mentioned that she would get back to me by Wednesday this week and if I made it through I would be assigned to a buddy. I am kind of confused because is this a first interview? what of the written problem solving test? Can anyone shed more light on this or has anyone experienced this? Much appreciated