iCall has announced iCall for the iPhone, an application that will purportedly allow you to seamlessly transfer incoming calls from regular cellular to WiFi, potentially saving plan minutes. The application integrates with the iPhone's Address Book and other phone-related functions.The company claims to have a working beta, but explicitly states on their website that no beta is currently available for review or testing outside of the company. However you can view a video demonstration of the App in action at the above link. The demonstration ironically takes place outside of a Starbucks store.
Additionally, according to the Web site, iCall also follows all rules for Apple's iPhone SDK and is not intended for use on jailbroken iPhones. Voice-over-IP is only available on Wi-Fi connections and will not be supported on the EDGE cellular connection, per Apple's rules for iPhone SDK developers. No mention of 3G is mentioned on the site, but one could assume that this rule will not change so that the cellular carrier's revenue stream is protected, regardless of whether or not 3G could handle the extra demand on bandwidth.
During a question and answer session following the iPhone Software Roadmap event on March 6h, a press member asked about the possibility of a VoIP (voice-over-IP) application for the iPhone. Steve Jobs left the door wide open, saying that while Apple wouldn?t allow such an application to access the cellular network, VoIP is certainly a possibility over WiFi.