Apparently Apple and AT&T have this agreement to block VoIP applications from using AT&T’s mobile network, however VoIP apps using WiFi are fine reports an article on zdnet.

This Apple/AT&T agreement comes about via a letter sent to the FCC by AT&T, and the letter states AT&T and Apple agreed that Apple will not take affirmative steps to enable the iPhone to use AT&T’s wireless service including 2G, 3G and WiFi to make VoIP calls without obtaining consent from AT&T.

Apparently without this agreement the price of the iPhone would have been higher than it is today. AT&T regularly review their policies regarding capabilities and features and will take a “fresh look” at possibly authorising VoIP capabilities for the iPhone on the AT&T 3G network.