"The National Telecommunications Security (NTS) Working Group (WG), formerly known as the Telecommunications Security Group (TSG), is the primary technical and policy resource in the U.S. Intelligence Community (IC) for all aspects of the Technical Surveillance Countermeasures (TSCM) Program involving telephone systems located in areas where sensitive government information is discussed. TSG Standards will be replaced by and issued as Committee on National Security Systems Instructions (CNSSIs). Implementation of CNSS instructions/TSG standards neither prevents the application of more stringent requirements nor satisfies the requirements of other security programs such as TEMPEST, COMSEC, or OPSEC. TSG Standard 1 is an introduction to telephone security that provides general information linked to the existing TSG standards and re-issued as CNSS instructions."
TSG Standard 1
Committee on National Security Systems: http://www.cnss.gov