They collect cell phone location data collected by all major cellular phone carriers, and then sell that location data to law enforcement agencies (no warrant needed). The data is used to conduct surveillance on potentially everyone in the U.S. who has a cellular phone.

Source: Senator Wyden Demands Answers from Prison Phone Service Caught Sharing Cellphone Location Data | Electronic Frontier Foundation

When the FCC mandated “wireless 911” technology so that phones would be required to report their position to 911 operators, I predicted this would be used for surveillance purposes and was, in fact, the primary goal.

Coldstreams