T-Mobile was found guilty by the Department of Labor for having an employee handbook with statements that discourage workers to unionize. It is illegal for employees to discuss wages, or complain about working conditions. The CWA had been trying to organize these employees when it discovered the violations in the employee handbook.
See "T-Mobile Found Guilty of Violating U.S. Labor Laws", Malathi Nayak, Business Insider, March 19, 2015