Google has accidentally let slip some secret plans relating to its Google Voice call management service, revealing that the application may soon be coming to markets outside the US.

US publication BusinessWeek managed to sneak a look at Google's " improperly formatted" PDF letter to the Federal Communications Commission (FCC), in which Google claims that the service has 1.419 million users, 570,000 of whom use it seven days a week.

The letter also reveals that Google has signed contracts with a number of " international service providers for inputs to Google Voice", hinting that the service may well go global shortly. Although it adds that none of these services has been launched yet.

The FCC launched an investigation into Google Voice earlier this month after AT &T and others raised concerns that Google is blocking certain calls in rural parts of the US, thereby undermining the principles of net neutrality.

The regulator then sent Google a letter asking it to explain how Google Voice works and why it is blocking calls, a practice prohibited by the FCC.

The service also came under the FCC spotlight in July, after allegations that Apple was deliberately delaying approval of the service for use on the iPhone, saying that the device may already have features that are very similar to Google Voice.