FaceTime on iMac doesn

On one of our iMacs, FaceTime does not send video out. It works within our home WiFi network, but not when talking to someone else. So the camera is working fine. I did a FaceTime from one of our iMacs to the one not sending video out, and it worked fine within our home WiFi, sending and receiving both audio and video. But when we call someone else from that iMac, they don't see any video. It works fine from our other iMac, so I don't think it's an issue with the ISP, unless somehow they're blocking a port that the particular iMac is using (although I have no idea how I could diagnose that). Any ideas and what it might be? Thanks.