Group FaceTime has been unavailable as a feature on all compatible devices since the FaceTime bug was publicized on Monday, January 28.
As Apple worked on a permanent fix for the bug, the company disabled the Group FaceTime feature to prevent it from being used.
The bug allowed iPhone users to exploit a Group FaceTime flaw that let one person connect to another person and her conversations (and see video, in some cases) without the other person ever having accepted the call.
Apple was first informed of the bug right around January 20 by the mother of a teenager who discovered it, but the company did not begin working on a fix until it went viral and spread across the internet.
Apple has since apologized and said that it is working on a way to better ensure that serious bug reports get to the proper people to prevent such a situation from happening in the future.
With the Group FaceTime servers back online, Group FaceTime is once again functional, but it is now limited to iOS devices that are running iOS 12.1.4 or later. The feature will remain disabled on devices running iOS 12.1.3 and earlier.