The National Security Agency started to delete malformed call detail records on May 23, this year, more than a month ago. Title V of the Foreign Intelligence Surveillance Act (FISA) and the USA Freedom Act of 2015 allow the intelligence agencies to collect call metadata related to certain types of calls involving persons of interest. The intelligence Agency also confirmed to have addressed the root cause of the problem for future CDR acquisitions. The agency reported the problem to the Congressional Oversight Committees, the Privacy and Civil Liberties Oversight Board, and the Department of Justice.”]
Source: https://securityaffairs.co/wordpress/74087/intelligence/nsa-deletes-call-detail-records.html