The way that these spammers have been working is by making a group call to a large number of people, and hanging up quickly afterwards. This initiates an endless string of callbacks when those included in the call begin to realize they've missed a call, and redial the whole group, and then quickly hang up as well when they realize their mistake. One user reported 59 strangers being included in a single such call.
Disturbingly, sometimes there would be no human on the other end of the video, only creepy art on the walls, or in one instance, a bathtub with running water. The seriousness of this issue can be compounded in households with children. There is no way of knowing whether these pranksters are predators, and children with a phone in their hand will most likely answer an incoming video call, putting themselves in potentially serious danger.
One strange thing is that FaceTime has a cap on the number of people who can participate in a group call at once, which is 32. The fact that multiple people are reporting 50+ members in these calls means that the perpetrators must be finding easy ways to bypass the limit, and Apple still hasn't taken any measures to fix that. To contact someone on FaceTime, only a phone number or e-mail are needed, and these can be easily compromised by an innocent mistake such as accidentally clicking a phishing link in an e-mail or text.
Numbers can only be blocked one at a time on FaceTime, and even blocked numbers can reappear in a group call started by someone else. Even worse, there is no way to limit incoming calls to contacts only, meaning that if you've been compromised and a victim to such spam calls, the only permanent solution may be to take a break from FaceTime.
The problem was first reported as a serious issue last year, with thousands of people confirming on Apple's forums that they are also dealing with the same problem. There is still no solution from Apple in sight, although it would hardly be difficult to think of one. Allowing blocking numbers en masse, and the ability to restrict calls to contacts should be simple enough—right, Apple?