The US Federal Communications Commission (FCC) has written to Apple demanding to know why it rejected the Google Voice for iPhone application, and removed related third-party apps from its iTunes App Store.

The letter (PDF) asks Apple to explain its reasons, and to clarify exactly which applications were removed. It also asks whether there are any differences between Google Voice for iPhone and any other VoIP applications which have been approved, and for Apple to explain its standards for approving iPhone applications.

"Recent press reports indicate that Apple has declined to approve the Google Voice application for the iPhone, and has removed related (and previously approved) third-party applications from the iPhone App Store," reads the letter.

"In light of pending FCC proceedings regarding wireless open access (RM-11361) and handset exclusivity (RM-11497), we are interested in a more complete understanding of this situation."

However, any response from Apple is unlikely to shed light on its apps approvals process, which has been a source of confusion and irritation for many developers, as the company is able to request confidentiality on certain information.

The FCC has also sent a similar letter to AT&T, Apple's exclusive carrier for the iPhone in the US, and to Google requesting more information on the matter.