Now you can comment on VoiceThreads using your phone instead of a microphone. While viewing a slide, there is a phone icon near the microphone icon. You can click on it, put your phone number in, and they call you to leave a message on the slide. The only problem I see with this right now is that you have to have registered an account with VoiceThread to use this feature. That is actually a good thing too, because it tracks who leaves the comments.
Check out this VoiceThread for an example on how to leave a voice comment using your phone: