Business Insider reports that FaceTime calls won’t use your carrier minutes, even if initiated during a voice call. It shouldn’t, as FaceTime requires Wi-Fi. The question was whether the voice call would be maintained in the background in case the FaceTime call drops. The answer is no.
People will complain that FaceTime is unavailable over 3G, but you can’t participate in a video call while doing many of the things that you do during a voice call. Plus, FaceTime over AT&T’s notoriously spotty network would be a disappointment. Over Wi-Fi, it will be fantastic and hugely popular.