We would prefer audio and video over the web. (Audio-only service is available from Skype / SkypeOut.) An ability to block incoming audio would be a plus, for those times when someone puts you on hold and do not realize that they have music-on-hold.

We do not need screen sharing. I guess it would be cool, but our presentations are brought in on a laptop and its video-out port connects to a video projector. My thoughts were along the line of pointing a webcam at the projection screen sometimes, pointing it at the presenter other times.