Well it seems as if Apple always has to deal with some
lockscreen bugs here and there. With Group Facetime
another security issue has become available. When in
the lockscreen, it is possible to view contact details.

The function is easy to reveal according to a video of Jose Rodriguez who has already revealed security flaws of Apple in the past.

With Group FaceTime and the invitiation of another person, it is possible to reveal the contact details of the contact to add to Group FaceTime.

This bug is active since iOS12.1 (newest version to date of November 5^th, 2018).

It is expected that Apple will fix that with iOS12.1.1…

Conclusion: Best thing is to disallow access of Siri when phone is locked – again!

Or: Do not give your phone to other persons!