Google had sent reply letter to FCC as Google was asked to give more details about rejection of Google Voice application for iPhone.
In the letter, Google asked to keep some portion of the letter as confidential.
But the secret information is made public as Freedom of Information Act request was filed.
You can read the Google's answer (un-redacted) in this pdf file which is published by FCC in its Website.
Find below the key part of the un-redacted portion of the answer.
Apple's representatives informed Google that the Google Voice application was rejected because Apple believed the application duplicated the core dialer functionality of the iPhone. The Apple representatives indicated that the company did not want applications that could potentially replace such functionality