To reveal the magician's secret: the lower tone indicates the row on the phone keypad, and the higher tone indicates the column. So on a keypad, the first four digits 1-5-7-0 consecutively goes down the keypad's rows and alternate between the first and second column, and you can see that same pattern in the picture.
IOW, each digit plays a two-note chord indicating the row and column on the keypad.
