Trying to please her

There were a few open books lying on the table. Carl Black was busy reading those books and making notes in his study. It seemed that he was lost in the world of books. Just then his phone started ringing. But he was so immersed in his works that he didn't look at the caller ID while he answered the phone "Hello."

There was silence on the other end for a few moments. He didn't care about it and kept the phone on the table without disconnecting it. He started working again as if nothing had happened. He even didn't glance at the phone. Several minutes later, the phone started ringing again. This time he didn't pick up the phone. The phone rang until the end of the ringtone and stopped. A few minutes later, the phone again rang. This time also he did the same thing. He answered the call without looking at it "Hello."

To his surprise he heard Lisa's irritated voice "Professor, are you so busy that you can't answer my call?"