AT&T plans to start testing a location-based service that would track a mobile phone user to prevent credit-card fraud. Privacy advocates want written guarantees that the collected data will only be used for that purpose. Credit-card companies could offer cardholders an app that gathers the information directly from the phone. The service would let them confirm when a cardholder lands in a foreign country to avoid wrongly denying credit. The U.S. Government Accountability Office found that the businesses did not have “consistently implemented practices to protect consumers’ location privacy””]