In its explanatory letter to the Federal Communications Commission (FCC), in connection with Apple's supposed rejection of Google Voice, AT&T has clarified that giving due to consideration to the risks the Apple and AT&T presumed in bringing the iPhone to market, both the partners had agreed to block VoIP apps from using AT&T's cellular networks.

Saying that, as per the arrangement, VoIP apps that use Wi-Fi were not to be blocked; AT&T noted that Apple's software development kit (SDK) enabled application providers to build VoIP applications using iPhone's Wi-Fi functionality, and that the App Store already features such applications.

AT&T elaborated in its letter: "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. AT&T and Apple also agreed, however, 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 further said that such a decision was taken after assuming the service revenues that iPhone users would help generate per month. Specifically, both Apple and AT&T needed to ensure that the revenue stream of AT&T's voice plans available for iPhone users should not be affected adversely by enabling VoIP calling functionality on the iPhone.