The Federal Communications Commission's probe was described to The Associated Press by people with knowledge of the inquiry. They spoke on condition of anonymity Friday because the FCC's letter had not yet gone to Google.
Google Voice lets people sign up for one number that can route incoming calls to cell, office or home phones. It also lets users place calls, including international calls, at low rates.
AT&T has complained that Google Voice blocks calls that AT&T and other phone companies are required to connect. Google had no immediate comment Friday.