iPhone 6 sometimes not ringing with iOS9

So my Problem is with iOS9, that my iPhone 6 is sometimes not ringing. Its sporadic and - i think - only with Numbers, that are in the Call History or under Contacts, then the iPhone is only vibrating verry short for one time, the Display is dark and it makes no noise. If i press the Home Button i can see who calls me.

So i think it must be a Bug, because in 95% everything works fine and with iOS 8.XX i never had this issue.

When i tried to reproduce the Problem, i called my self a few times and sometimes it was ringing normally, and a few time i had the issue that there was only a short vibration with a turned off display.

I reseted all Settings a few times, but it did not help. Anyone had the same Issue?