The technology company made the claim in a in response to enquiries about the process of accepting and rejecting iPhone apps, brought about by a complaint from Google and AT&T.
In the letter, Apple says that the Google Voice application continues to be studied but that it has not yet been approved because "it appears to alter the iPhone's distinctive user experience by replacing the iPhone's core mobile telephone functionality and Apple user interface with its own user interface for telephone calls, text messaging and voicemail".
It added that it had not been in contact with AT&T, which is the sole mobile network to sell the iPhone, over whether or not to approve the Google Voice application. AT&T confirmed this.
In the letter, Apple also revealed some detail about how applications are approved for its iPhone App store. It says that it has approved 65,000 applications so far and that 1.5bn apps have been downloaded. While app developers complain about waiting for long periods to find out if their products have been accepted, Apple said that 95% of applications are approved within 14 days of their submission.
Google and Apple are increasingly moving in to competition with each other. While Apple has the iPhone, Google has developed its own mobile, Android, and is also working on its own computer operating system.
Google has also been questioned about its approval process for Android applications. It told the FCC that it relies on the users of its Android Market -- the Google version of the App Store -- to flag up applications that violate its terms.