What did Oskar Schell do with the phone after his father's call on September 11, 2001?

After Oskar's father calls from the World Trade Center, Oskar replaces the phone that has the messages from his dad with another phone he buys at Radio Shack and keeps the old phone under his bed, where he pulls it out and listens to one of his dad's messages when he wants the sound of his dad's voice.