The U.S. Federal Communications Commission has written to Apple, AT&T and Google questioning the rejection of Google Voice and related applications from theApp Store.

In a letter sent Friday to Apple, the agency asked the company why Google Voice was rejected, which related applications have been rejected along with it, and what role AT&T may have played in the decision. It also asked what the difference is between Google Voice and other VoIP (voice over Internet Protocol) software that has been approved for the iPhone.

More broadly the agency asked Apple what other applications have been rejected for the phone and why.

Apple’s process for approving or rejecting applications for the iPhone has drawn wide criticism for being murky and unpredictable. Google Voice is a VoIP service that lets users set up one phone number and have it ring on all of their phones at once, and control which phones ring at which times of the day or week, among other things.