Responding to popular unease at the idea of being located by cell phones, Verizon has decided to place warning labels on the phones it sells to warn people that they can be tracked.

In a letter to Congressmen Ed Markey (D-Mass.) and Joe Barton (R-Texas) Verizon answered several questions about the way the company uses location data.

The warning labels say This device is capable of determining its (and your) physical geographical location and can associate this location data with other customer information. To limit access to location information by others, refer to the User Guide for Location settings and be cautious when downloading, accessing or using applications and services.

Beyond the warning label, Verizon answered several questions from the Congressmen from earlier letters they had sent to the carrier. Verizon said that most location-based services require a user to opt-in before they engage the services.

The company also said it stores customer data for a period of about seven years.

Mobile phones generally track user's location via the cell towers; the phone has to signal a tower in order to decide where to send the data or voice transmission, as well as for calculating the handoff between towers. Law enforcement agencies can get the information with a subpoena, though often it isn't necessary as phone companies generally cooperate with those requests. The trail of phone signals would not give an exact location, but it will be close.

This is somewhat different from what Apple was doing. Apple's gathering of cell tower and local Wi-Fi hotspot data was designed to build a map of local points of transmission, so that the iPhone could send the data to Apple. Combined with the data from thousands of other customers, the system would build a map that would allow apps that need the location information to work much faster, as they wouldn't have to compile the data afresh each time they were used.

The other big difference between the location data stored by Verizon is that it doesn't store the information locally, and it isn't accessible via the phone. Apple's iOS drops a copy of the location file onto a users' computer every time it is synced with the phone. Google's Android operating system also stores a file with the last 50 locations (or 200 Wi-Fi hotspots) in it. In both cases a hacker could conceivably access the file.