In my case it was a consultant that interviewed me on the phone. She also spent like 15 minutes telling about herself and her projects so I get an idea about the job. So, I think the case took like around 20 minutes. As said, it was really simple, she basically asked me to solve a breakeven point. I also did not have the feeling I had to rush.
In my opinion, the first phone interview is a superficial screening and what is important is to make rapport with the interviewer. Like being nice and be interested on her experiences. Over the phone they assess if you are a nice person and do not assess problem-solving skills in much detail.