Federal regulators Wednesday approved a plan to create a nationwide emergency alert   system using text messages delivered to cell phones.  Text messages have exploded in popularity in recent years, particularly among young people. The wireless industry's trade association, CTIA, estimates   more than 48 billions text messages are sent each month.  The plan comes from the Warning alert and Response Network Act, a 2006 federal law that requires improvement to the nation's emergency alter system. The act tasked the Federal Communications Commission (FCC) with coming up with new ways to alert the public about emergencies.  "The ability to deliver accurate and timely warning and alert alters through cell phone and other mobile services is an important next step in our efforts to help ensure that the American public has the information they need to take action to protect themselves and their families before, and during, disasters and other emergencies," FCC Chairman Kevin Martin said following approval of the plan.  Participation in the alert system by carriers -- telecommunication companies -- is voluntary, but it has received solid support from the wireless industry.  The program would be optional for cell phone users. They also may not be charged for receiving alerts.  There would be three types of messages, according to the rules.  The first would be a national alter from the president, likely involving  a terrorists attack or natural disaster. The second would involve "approaching threats," which could include natural disasters like hurricanes or storms or even university shootings. The third would be reserved for child abduction  emergencies, or so-called Amber Alerts.  The service could be in place by 2010. The carriers' participation in the system is determined by   _  .
the US federal government mobile phone users the carriers themselves the law of the United States