https://www.cnet.com/news/apple-release-update-to-fix-group-facetime-eavesdropping-bug/

Last week, Apple turned off Group FaceTime after a bug was identified that allowed iPhone users to call another device via the FaceTime video chat service and hear audio on the other end before the recipient had answered the call. It essentially turned any iPhone into a hot mic without the user's knowledge. 

Apple on Friday said it'd fixed the vulnerability on its servers and that it'd issue a software update to re-enable Group FaceTime. Apple also apologized to users who were affected and said it takes the security of its products "extremely seriously."