There are three red phones throughout the pentagon that ring and flash. The first is located where you first start after opening a set of doors in corner. Second is on the catwalk circling the room after leaving the first elevator. The third is in one of the rooms after leaving the second elevator on the floor the power is on. Just listen for a cell phone type ring and follow it to the red rotary phone. Look at them and hold X till the ringing stops and you hear a busy sound. Once you get the third the music starts.