Every year, more than 50 million 911 calls are made from mobile phones in the United States, according to the National Emergency Number Association. But unlike 911 calls placed from traceable landlines, wireless calls do not provide emergency call center operators with location information-a shortcoming that can hold up emergency responders. To avoid such delays, the FCC initiated a plan requiring wireless carriers to provide call centers with callers’ geographic coordinates, their mobile-phone callback numbers, and the locations of the towers or antennas receiving their calls, by 2005. Experts estimate that it will take longer for the nearly 6,000 U.S. emergency call centers to upgrade their technology to accommodate the two methods of location tracking-handset-based assisted GPS and network-based tower location-used by wireless carriers. Here’s how it will work.