The application is already available for a number of smartphones
US telecoms giant AT&T has admitted that it struck a deal with Apple to prevent iPhone applications from using its network for VoIP.
VoIP allows voice calls to be routed over mobile internet but reduces a network operator's voice call revenue.
Apple and AT&T both responded to a federal inquiry into Apple's failure to approve Google Voice, a VoIP application for the iPhone.
Apple said that the app had not been rejected but is still under review.
In July, Apple rejected Google's application for the Google Voice app to be sold in its App Store, and removed several third-party applications that employed the same technology.
The app is already available on Blackberrys and phones that run both Google's Android and the Symbian operating system.
Apple's move prompted an inquiry from the US's Federal Communications Commission (FCC).
In AT&T's response to the FCC, the firm said that "AT&T and Apple agreed that Apple would not take affirmative steps to enable an iPhone to use AT&T's wireless service (including 2G, 3G and Wi-Fi) to make VoIP calls without first obtaining AT&T's consent".
However, it added that the firms had agreed that "if a third party enables an iPhone to make VoIP calls using AT&T's wireless service, Apple would have no obligation to take action against that third party".
AT&T argued that the deal was struck because "both parties required assurances that the revenues from the AT&T voice plans available to iPhone customers would not be reduced by enabling VoIP calling functionality on the iPhone".
In its letter to the FCC, Apple said that several VoIP applications such as Skype had already been approved for use over wi-fi wireless networks, rather than over the cellular network.
However, it added that the Google Voice app was still under scrutiny because it could "alter the iPhone's distinctive user experience", but added that Google Voice could be accessed via the iPhone's web browser.
In its reply to the FCC inquiry, Google argued that "the Google Voice features accessible by the mobile website are more limited than those features found in the App Store version".
Those features include direct access to the phone's contact list, or dialling directly from the application.