WASHINGTON, Aug 18 (Reuters) - The U.S. Federal Communications Commission (FCC) said on Wednesday it is investigating a T-Mobile US Inc (TMUS.O) data breach that the company said impacted more than 40 million former, prospective and current customers. "Telecommunications companies have a duty to protect their customers’ information. The FCC is aware of reports of a data breach affecting T-Mobile customers and we are investigating," an FCC spokeswoman said.