Oh hey I just tested on my MI 8 and it worked like supposed to, can't make phone calls.
Mi 8, stable 10.3.3.0, there is a bug with the airplane mode. When it is turned on, the phone can still make phonecalls and has data connection. Not very safe specially if you are on a plane.
I opened a thread but no one seems to have same problem.
Hope it's ok to post here