Dan Goodin, Ars Technica:

FaceTime users are getting bombarded with group calls from numbers they’ve never seen before, often as many as 20 times in short succession during late hours of the night.

Griefers behind the pranks call as many as 31 numbers at a time. When a person receiving one of the calls hangs up, a different number will immediately call back. FaceTime doesn’t have the ability to accept only FaceTime calls coming from people in the user’s address book. It also requires that all numbers in a group call must be manually blocked for the call to be stopped.

If this is happening to you:

A user can also turn off FaceTime in iOS settings or in the macOS app, but that prevents users from receiving wanted calls as well. Last, people can uncheck their phone number under the FaceTime setting “where you can be reached.” Once again, however, this will prevent wanted calls that are initiated using the user’s number.

They’ve found a hole in the system. Hopefully, Apple will roll out a fix before this grows much larger.