An article posted to the online outlet Motherboard has alleged that the carriers T-Mobile, Sprint and AT&T have been selling their US customers' geolocation data to private companies, who then went on to sell it to other parties such as those who issue bail bonds. The carriers in question have subsequently committed to put an end to these practices.
https://www.notebookcheck.net/Motherboard-report-implicates-major-US-carriers-in-a-geolocation-data-selling-scandal.393851.0.html