to also shed some light for following readers: I recently had something quite similar. In my case, it was an invitation to do participate in a "trial case interview" at McKinsey & Company.
I was being told that this is a possibility for me to assess what to expect in the real interview, but I also heard from a lot of friends that this might be a pre-assessment of your skills in order for them to assess whether you are "worth the consultant's time" in the real one, or not.
I suppose that you are applying in Germany? I heard from a couple of friends that they had a phone interview like yours, and I also talked to a Recruiter of McKinsey & Company in Germany. They told me that this is a "real round 1 interview" and you will be invited to the in-person interviews, if you can perform as expected.
So I can recommend to learn and act, as if this is the real one.
However, in any case: Relax and sit back, since this one will be in a comfortable atmosphere.