During an emergency, a 911 attendant can track a caller's phone using GPS. However, it is illegal for ordinary civilians to use this technology because it constitutes an invasion of privacy. In order to determine the exact location of a cell phone, you must have the consent of the cell phone's owner. If you simply wish to determine the general location of a cell phone, though, you can use several free resources available on the Internet.
Install a GPS locating device. Verizon and Sprint offer software that you can install on your phone or your child's phone in order to track it, and several websites offer this software for order online. Read reviews of the software before you purchase it; some websites may be scams or ineffective. This method will not tell you the location of any mobile number, only the mobile numbers registered with your software.
Download a GPS mobile application, such as MapMates, Google Latitude or GPSEd. These applications allow you to share your own location and see the location of your friends who are also have the application. These applications cannot tell you the location of a number that does not have the application or that has privacy settings which will not allow non-friends to see it.
Search for the phone number on the web. The area code will give you a general location, at least for where the phone was issued, and the number may be posted on social media websites which also include location information.
Check an online cell phone directory or a reverse lookup site. Many of these sites charge a fee, and they cannot tell you the real-time location of a mobile phone---only where the cell phone was issued. Moreover, several sites are inaccurate or merely scams.
Be wary of scams; most websites that profess to track mobile numbers can do no such thing. Tracking the real-time location of a cell phone is illegal unless you are an emergency attendant or a government official.