The sound you hear when you dial someone prior to them answering the phone. In 2005, ring tones pulled in $600 million, 20% ahead of estimates and more than double the 2004 take (USA Today 1/25/2006)  The research group IDC states that the U.S. ringtone market revenue opportunity is more than $1 billion by 2007 and that the total volume of ringtone sales will approach the 100 million mark by 2009.