Everybody has a cell phone, whether its an Apple iPhone or a Samsung Android. The cell phone providers, including Verizon Wireless, Sprint, T-Mobile, AT&T or Cricket Communications always know where your cell phone is located. When your cell phone is on and engaged in activity it connects to a cellular network through its towers or cell sites. The cellular network has the capability to capture the approximate location of the phone from a particular tower. The data included in these reports can be used to determine the specific cell tower being used and depending on the cell phone provider the approximate distance of the phone from the tower. Generally a cell tower is divided into three sectors at approximately 120 degrees each. The technology can usually place a cell phone within a several mile radius, depending on the number of cell sites in the area. Obviously the area will be much smaller in New York City than rural Montana.
When you make a call, or text or your cell phone contacts the internet for any reason, i.e. to check Facebook or Twitter, listen to Pandora Radio or get driving directions from MapQuest or WAZE, your cell phone provider knows the cell site your phone connected to. They will give that information to the government.
A cell phone company or representative or local detective or an FBI agent with the Cellular Analysis Survey Team (CAST) will determine your approximate location and testify in Court.
Every call is recorded by the date and time it connected with a specific tower or cell site. Some calls are transferred from cell site to cell site. A cell site can be a tower or on a building or other structure.
The phone companies have records of the locations of all cell sites and know which of the three sectors the call was connected to. In other words the company knows which side of its cell site your phone has located to. The above map is an example of a cell tower imposed over a map. When the cell phone contacted the tower it most likely was within the triangle on the map. Although, a cell phone does not always contact the nearest cell site, it will use a cell site in close proximity.
In every case I have tried involving the location of a cell phone at a particular date and time, the cellular companies information was generally accurate. In one case, the cell phone was tracked travelling over 20 miles of roadway by an FBI expert who examined the cell tower/sites it connected with.