https://blog.wongcw.com/2018/10/28/ios-12-1%e5%bc%95%e5%85%a5facetime%e7%be%a4%e8%81%8a-%e6%97%a7%e6%9c%ba%e5%9e%8b%e6%9c%89%e9%99%90%e5%88%b6/
iOS 12.1引入FaceTime群聊: 旧机型有限制